Roskilde is in West Zealand, Denmark, 35 km west of Copenhagen. Roskilde is an ancient city dating from the Viking age. The most important historical sights are the Viking Museum and Roskilde Cathedral. Roskilde is also home to the giant rock music event, Roskilde Festival.
Roskilde is south of Roskilde Inlet with the Viking Museum and some of the accomodation and restaurant options nearby. Central Roskilde is 1 km further south centered around the pedestrian street Algade/Skomagergade and Roskilde Cathedral. The city centre is quite small at around 1 square kilometer bordered by a ring road. The train station is at the southern end of the city centre. Roskilde Festival is 4 km south from the train station along Køgevej.
Roskilde was established more than 1,000 years ago, at which time it had a wooden church and a royal farm. In the 12th century, Roskilde Cathedral was built and Roskilde became the seat of the bishop and was also granted the status of a market town. The town was one of the most important in Denmark for the next few centuries until the reformation ende the importance of the church.
From Copenhagen Central Station (København H) it is about a 25 minute trip. Nearly all trains leaving Copenhagen besides those leaving for Elsinore or Sweden stops in Roskilde, but you would normally have to pay an extra fee if you jump on a Intercity train (i.e. trains to Jutland). Regional trains bound for Odense, Kalundborg or Nykøbing Falster are your best choices (they are marked with a green "Re" sign on platform screens). There is also a small branch line with occasional trains to Køge.
Intercity buses to and from Jutland lines 882 to Fjerritslev and 888 to Århus/Aalborg, makes stop at the Q8 gas station at Darupvej 2, by the highway. It is quite far from the city center, but there are several bus lines connecting from Roskilde Station, e.g. 600S
Important regional buses includes 600S which runs an arch around Copenhagen proper from Hillerød in the north (Frederiksborg castle) to Hundige Station in the south. 233 runs between Roskilde and Lejre Forsøgscenter, 234 connects with Slagelse and finaly 229E and 239 with Frederikssund.
Roskilde Airport (IATA: RKE, ICAO: EKRK)  is Denmark's premier General Aviation facility with planes taking off or landing, every 5 minutes througout the day. It is also the main base for un- or semi scheduled taxi flights to the Kingdoms more remote islands; Copenhagen Airtaxi  offers flights to Samsø, Læsø and Anholt for between 1,000-1,500 Kr, and more or less works as a regular airline, with flights bookable on the company website. Other options include: Samair to Samsø and Starling to Ærø.
You can easily walk from the train station to the main square with the cathredral and from there to the Viking Ship museum.
There are a few local buses running from the train station.
Parks and nature
The legendary Roskilde festival  is still going strong despite 40 years of sex, drugs and rock'n'roll! started by a group of friends in 1971 inspired by the Woodstock festival, it has grown from a few thousand guests in its first year, to over 115.000 visitors from all over the world, and is routinely sold out with over half the tickets sold outside of Denmark.
The festival takes place in late June early July on Roskilde Dyreskueplads south of the city. To Get in there are several options; from Monday-Sunday during the festival trains depart from Roskilde Station to the festivals very own station inside the camping area (West) every 30 minutes for 20 kroner. There are also shuttle buses from Roskilde Station to the camping area (East) all day during the festival. When arriving at the festival you exchange your ticket to an armband/bracelet which gives you access to both the camping area, and the stage area.
Many people show up a week before the music starts to get their tents up so they can sleep close to the stage area, and limit walking as much as possible as the camping area is huge! A good tip is securing a place as far from trees and fences as possible, since they in essence become one big urinal during the festival, if rain is forecast, you should also attempt to camp as far from the marked walkways, preferably on a slope, since mud quickly becomes and issue.
On the camping grounds there are several service areas with food stalls and supermarkets to stock up with eat & drink, some also have first aid, toilets and showers. Inside the stage area there are also many many restaurants and bars - most of them with a theme, and some of them are really huge and elaborate. There are also many shops where you can buy necessities and other rubbish.
Music is playing from Thursday - Sunday and are usually varied, with some international headline acts, and much experimental and indie music and the smaller stages.
On Roskilde festival the nearest supermarket is Fakta at Køgevej 108, about 1300 meters (a 15-20 minute walk) due north (past the highway) from the eastern entrance. The downtown area which has a pedestrianized area full of different shops is a further 1½ km away (or 30-40 minutes by foot).