View over I-395 towards the Pentagon, with the Washington Monument in the background
Northern Virginia is highly populated, and smashes most all stereotypes of Virginia, that one-time Southern state. Though much of it is considered a suburb of metropolitan Washington, D.C., the inner suburbs are really cities in their own right, with world class attractions and nightlife, and the ensuing sprawl is likewise dense with its own attractions and culinary gems.
Alexandria While bigger than just its historic center, Old Town Alexandria is a great tourist destination for shopping, dining, boating, and colonial history.
Arlington Were it not known popularly as southwest D.C. (which it historically was), and as a county rather than a city (which it basically is) Arlington would be the third largest in the state, ahead of Richmond, and potentially first in cultural vibrancy.
Fairfax County The second tier suburbs of Fairfax, the state's most populous county by far, are home to enormous Asian immigrant communities (and correspondingly great food), as well as George Washington's estate and the area's largest shopping mall.
Loudoun County Northwest of Fairfax, Loudoun is more rural in character, but a well-trafficked county nonetheless for Dulles International Airport and its outlet shopping.
Stafford County Stafford County and its overwhelmingly rural exurbs... you mostly drive through.
Prince William County Home to Bull Run National Battlefield, Quantico and the National Marine Museum, and Potomac Mills Outlets.
Springfield — not a fabulous destination—more of an urban/suburban mess—but there are still some really great Asian and Latino restaurants sprinkled throughout all the strip malls.
Sterling — home to ever busy, beautiful, and impractically-designed Dulles International Airport, as well as the National Air and Space Museum's big annex.
Vienna — Sachertortes, great beer, mountain scenery, skiing, lederhosen, the waltz—wait, just kidding! But it's home to the big Wolf Trap Performing Arts Center, and is right next to Tysons Corner Mall.
Northern Virginia is emphasis on the Northern. No drawls here, the politics are liberal, and people aren't too sure what to make of grits. Actually, most Northern Virginians were born elsewhere, with nearly half of them either from a different country or born to immigrant parents. Outside the immigrant populations, which are fairly wealthy in its own right by any national standards, the native-born population is downright spectacularly wealthy. Polo shirts and khakis shorts rule, pearls adorn the night, mansions are so nouveau-riche (and prevalent), and world class golf is in every direction.
Northern Virginia has always been closely tied with the nation's capital. This part of the state benefits from the history and the cultural aspects of Washington D.C., featuring famous museums, cemeteries, and the home of the first president of the United States. Arlington and Alexandria, in particular, are every bit as dense urban areas as D.C. itself—if not more so. Fairfax County is only slightly less dense, and, with over one million residents, is the most populous county in the state by far.
Much of the northeastern corner of Virginia, aside from the cores of Arlington and Alexandria, was farmland for most of its history until the period immediately following World War II, when government employment increased and the population around Washington D.C. began to grow. The area experienced another explosion in growth due to tech industry jobs in the early 90s. Today it remains one of the fastest growing areas of the country. While Northern Virginia continues to expand, the region ranges from crowded planned cities with excellent shopping to soccer-mom suburbia, from ethnic neighborhoods full of authentic restaurants to the upper-crust style of the Hunt Country.
Washington Dulles International Airport (IATA: IAD)
Ronald Reagan Washington National Airport (IATA: DCA)
Leesburg Municipal Airport (charter and private planes only, no commercial service)
Baltimore/Washington International Airport (IATA: BWI)
Northern Virginia has two big airports: Ronald Reagan NationalIATA: DCA in Arlington, and Washington Dulles InternationalIATA: IAD in Sterling. Baltimore-Washington International IATA: BWI is often cited as the area's third airport, but if you factor in the $100+ long cab ride, that Southwest Airlines flight isn't as good a deal as it seemed, right?
Private and charter craft can also land at Leesburg Municipal.
A good long list of major interstates lead into Northern Virginia. I-495 (the Capital Beltway) and I-95 both lead into Arlington and Alexandria from Maryland, with I-395 being a special extension of I-95 at the Beltway into Washington, D.C. from the south. I-95 keeps heading south to Fredericksburg and on to Richmond. On the Maryland side of I-495, you can pick up I-95 north to Baltimore, Philadelphia, New York City, and on to Boston, or I-270 to I-70 to Pittsburgh and on to the Midwest.
I-66 (paralleling US-50) comes in from West Virginia and the Shenandoah Valley via Front Royal, west of which it connects with I-81. Note the highway east of I-495 is HOV-2 (two people per car except motorcycles) only in the direction of rush hour, 6AM-9:30AM and 4PM-6:30PM.
Amtrak provides train service on a regular schedule between points north and Washington, DC's Union Station. Washington DC's mass transit (WMATA) provides service to Northern Virginia on the Blue, Yellow, and Orange lines.
From the south, Amtrak provides twice daily service from Newport News, Virginia with a last stop in Alexandria prior to arrival at Union Station.
Taking Metrorail might be your best option - there are many stations within the Beltway (near Washington, DC, in other words), including one that emerges right outside the Fashion Centre at Pentagon City. It is also the easiest way to enter Washington, DC.
Driving can be convenient, unless you wish to enter Washington, DC, or cross the Beltway in either direction. The highways (I-395, I-95, I-495, and I-66) are extremely backed up during rush hour. The "Mixing Bowl" (the intersection of I-395, I-95, and I-495 south of Washington, DC) is especially notorious. Attempting to travel by road between 2:00 PM and 6:00 PM on any weekday (and, for that matter, on a weekend) is strongly discouraged.
Sunset parade at the U.S. Marine Corps War Memorial in Arlington
Arlington National Cemetery, Arlington, Virginia. Just across the Potomac River from Washington D.C., adjacent to the Pentagon. Closes at dusk. This national military cemetery includes John F. Kennedy's tomb and the house of General Robert E. Lee. Visitors can watch the changing of the guard ceremony in front of the Tomb of the Unknown Soldier. 
Old Town Alexandria, Alexandria, Virginia. This highly walkable Old Town at the edge of the Potomac River features historic buildings, churches, museums and art galleries, a farmers market, and a variety of places to eat and shop.
Mount Vernon in Mount Vernon, Virginia. George Washington lived in this country estate which overlooks the Potomac River. 
National Museum of the Marine Corps in Quantico, Virginia. Newly revamped museum displaying the history of the Marine Corps and their actions around the world. Features several aircraft, weapons, and interactive displays. 
National Rifle Association Museum and Headquarters in Fairfax.
Pentagon. Just across the Potomac River from downtown DC. While lingering is not recommended for security reasons, you should know it is the largest office building in the world, and covers 4 zip codes. (Army, Navy, Air Force and Department of Defense.)
Steven F. Udvar-Hazy Center - National Air and Space Museum, 14390 Air & Space Museum Pkwy., Chantilly, ph: (202) 357-2200. Located near Dulles International Airport, this museum houses many air/spacecraft, including the SR-71 "Blackbird" spy plane, the Concorde supersonic jet and the space shuttle "Enterprise". Parking is available for $15/vehicle. Additionally, a shuttle is available from the Air and Space Museum downtown. Prices range from $5 to $7 depending on number of tickets bought. 
Great Falls Park, in McLean. Gorgeous national park with waterfalls and hiking trails, minutes from the beltway. Kayaking and rock climbing. Going to the park after a large rain storm provides different views as the water levels can change drastically.