There are several forms of Frisian, but the most widely spoken of these is West Frisian, or Frysk. It is a language spoken mostly in the province of Friesland (Fryslân) in the north of the Netherlands. West Frisian is the name by which this language is usually known outside of the Netherlands, to distinguish it from the closely related Frisian languages of Saterland Frisian and North Frisian, which are spoken in Germany. Within the Netherlands, however, (and often in other countries) the West Frisian language is the language of the province of Fryslân and is virtually always just called Frisian: Fries in Dutch, and Frysk in Frisian. The 'official' name used by linguists in the Netherlands to indicate the West Frisian language is Westerlauwers Fries (West Lauwers Frisian), the Lauwers being a border stream which separates the Dutch provinces of Fryslân and Groningen.
Most speakers of West Frisian live in the province of Fryslân (Friesland in Dutch) in the north of the Netherlands. The province has 643,000 inhabitants (2005); of these, 94% can understand spoken Frisian, 74% can speak Frisian, 65% can read Frisian, and 17% can write it. For over half of the inhabitants of the province of Fryslân, 55% (c. 354,000 people), Frisian is the native tongue.
To find out more about West Frisian, including the history, visit Wikipedia.
West Frisian is similar in both sound and grammar to Dutch, Afrikaans and some northern dialects of German. There are two grammatical genders, common and neuter, although they are nowadays very corroded and usually always the same in the plural.
It is the closest living language to English and shares mutual intelligibility with both Dutch and English, although Dutch and English only share, at most, a basic level of intelligibility with each other.
Similarly to the situation of Dutch, West Frisian is hard to learn, because Frisians usually speak English very well, so that a Frisian might respond in English if he sees that you're having a hard time with the language. Additionally, since Frisian is a minority language, educational material might be somewhat hard to come by. Furthermore, all Frisians also speak Dutch and if you travel to the Netherlands it is likely you'll focus on your Dutch first, making knowledge of West Frisian somewhat redundant.
A lot of Frisians speak some form of "Interference Frisian" in which they use a lot of Dutch words in their speech. If you'd study this list thoroughly and remember each and every word on it, you still might find that a Frisian uses a certain amount of Dutch words instead of words on this list. A Frisian who speaks correct Frisian is said to speak "geef Frysk".
All road signs are in Dutch anyway (if they don't use symbols) and most shops or other businesses will use Dutch signs, however, occasionally you might find a "smûk" gift shop, restaurant, or bed and breakfast (bêd en brochje) with Frisian instead of Dutch signs.
Also note that there are quite some phrases on this list which are "just translated", but which would be very weird to actually use, due to cultural differences. In some cases it is explicitly stated that and why a certain phrase is never used.
Despite its relatedness to English, Frisian might be very hard to speak for an Englishman due to it's large number of vowels (much larger than Dutch) and includes all the same sounds which can be found in Dutch and which are hard to pronounce for most Englishmen (g/ch, ui, eu, uu, etc.).
Frisians also use a lot of understatements so don't assume that a Frisian doesn't like something if you don't sense any enthusiasm. "It moat mar" literally means something like "It has to be (it's not like we have any other options)", but which actually means "This is only a little bit short of our extremely high standards, so let's take it". Another example: a phrase like "Noh, bêst genôch, no?" literally means "Well, this is good enough, right?" but actually translates to "This is f*ckin' awesome." Like other Dutch people, Frisians are also very honest and direct. When a Frisian doesn't like it (s)he will just flat out tell you "Ik fyn it mar neat", meaning "I don't like anything about it at all".
West Frisian vowels are similar to Dutch although there are a few substantial differences; for example, accented vowels like â are considered separate letters in West Frisian, not simply a variation of a. Also, there a few sound differences in consonants.
The letters 'q' and 'x' do not appear in Frisian except for names; the same applies to the letter 'c' when not part of the combination "ch".
like 'b' in "bat" or like 'p' in "map" at word end
like 'd' in "day" or like 't' in "tap" at word end
like 'f' in "fire"
like 'g' in "green" or sometimes like Dutch G, dragged from the throat
like 'h' in "hot", silent when before j or w
like 'y' in "yes"
like 'k' in "kit"
like 'l' in "lock", silent between â and d or t
like 'm' in "moon"
like 'n' in "now" or like 'm' when before the letter p, nasalises preceding vowels when put before s, z, f, v, w, j, l, r and disappears. 'ins' will sound as one single, strange, vowel.
like 'p' in "pen"
usually silent at the end of the first word in a compound, at the end of a word which is not the last word in the sentence, otherwise usually a single rolled short r, sometimes like in English or French.