Basic Info

Like the guitar and bass the banjo is also a transposing instrument; it's still written in the key of C but every note is written one octave higher than it sounds. The banjo is not a overly common instrument in jazz or contemporary music (and probably will never be in a classical ensemble) but there are few genres where you will encounter it especially since it has seen a small rise in popularity in recent years (for example Bela Fleck and the Flecktones)
Types
5 String Banjo

The 5 string banjo is probably the most common variation of the instrument and is most common in bluegrass, folk, country and some jazz music. What differentiates this banjo from a tenor (below) is the addition of the 5th "thumb string" that is more of a percussive tool tuned to one static note and not usually fretted.
4 String (Tenor) Banjo

The tenor banjo is a slightly shorter 4 string version of the banjo that was common in early swing and jazz music before the introduction of the guitar to the genre. Still used today in a variety of styles and most banjo players are capable of doubling on either version of the instrument.
Range
5 String Banjo
Regular Range

In normal tuning the banjo can play a D3 as its lowest note although remember that this note is written as an D4 in the treble clef as the banjo is written an octave higher than it sounds. A 5 string banjo will usually have 22 frets so it is usually safe to write a C7 but anything higher should be checked with the player/instrument.
Extended Range

In certain genres and pieces the 5 string banjo may be in "C tuning" where the lowest D string is tuned down a step to a C4 (written). This is a specialty tuning and the standard "open g tuning" is much more common.
4 String Banjo
Regular Range

The standard tuning for a 4 string banjo is known as "C tuning" or "tenor tuning". This tuning (CGDA) has the instrument's lowest D string tuned down to a C giving it a slightly lower range (although the same as the 5 string if tuned in a similar way).
Articulations
Articulations
Staccato

On the banjo a staccato is played usually by lifting the fret hand finger up early, cutting off the length of the note. This can be done any number of times in a row; legato markings should be used for notes in a staccato passage that need to be played for their full value (see below).
Tenuto

Tenuto markings on the banjo are best used when a note surrounded by staccato markings needs to be played for its full value. On its own this marking denotes that the note should receive some emphasis although what form that will take is contextual and may be left up to the player (unless specified otherwise).
Accent

Accenting a note on the banjo is achieved usually by plucking/picking the desired note harder than usual. Can be used on both full chords and single notes.
Marcato

As with jazz arranging in general the marcato articulation usually results in an accented note with a shorter value (about 2/3) than a standard accent. Use for short, punchy lines or chord shots that need some space and tightness to them.
Slur/Legato

A slur on the banjo can be played in a number of ways: a hammer-on/pull-off (most common), a slide or a bend (less common). Using a slur articulation without specifying which technique to use will leave the player to decide which is best to use in the context of the song/phrase. Entire slurred phrases on the banjo can also be notated with a legato marking although a slur should still be used. See below for more information on each separate technique.
Hammer-On

A hammer-on is achieved by using the fret hand to "hammer" onto the neck of the banjo, producing the desired note. Because this note is not being picked by the other hand its volume is slightly lower than that of a picked note with each successive hammer-on being slightly quieter than the previous note. Long passages of hammer-ons are almost always played by incorporating pull-offs into the playing as well (see below).
Pull-Off

The opposite of a hammer-on, the pull-off is achieved by picking a note then "pulling off" the finger either to an open string to to another fretted note below. Long passages involving either hammer-ons or pull-offs typically incorporate both techniques to keep the volume and rhythm consistent. In a nutshell: a hammer-on is for ascending movement and a pull-off is for descending passages but expect some combination of both if the passage is longer than one or two notes.
Effects
Special Effects
String Bend

A common string instrument effect where the string is pushed up or down to increase its pitch. Bends should be notated either by using a grace note if the note is bent immediately or using normal bend notation if the bend is in time. The first note of the bend should be the actual note being played with the second note being what is being bent up to; typically a whole step bend is the recommended maximum although a minor 3rd bend can also be possible in the higher register.
Slide

A slide is a common string instrument articulation/effect that involves picking a note (or group of notes) and physically sliding the fret hand finger up or down towards the next note. As mentioned above a written slur might result in a side in certain contexts but to specify one the standard slide notation can be used.
Vibrato

Vibrato on the banjo is achieved by moving/bending the fret hand finger up and down slightly to create subtle pitch fluctuations in the held note. This can be done slowly or quickly for varying different types of vibrato; this effect should be notated either with a vibrato marking or with a vibrato expression in the case where another articulation is already being used.
Trill

A trill on the banjo is accomplished by rapidly hammering-on and pulling-off between two notes in rapid succession. Unlike some other instruments a trill can be played as high as a major third without too much difficulty. Usually by default the trill will either be a whole step or the next logical note (next note in the key or altered note in the bar), accidentals can also be placed above the trill marking to specify which note is to be played.
Fret-Hand Mute

There a few possible types of mutes on the banjo but the most common in jazz and contemporary music is the fret-hand mute (sometimes called the left-hand mute but this is inaccurate as not all banjo players are right-handed). This effect is achieved by preventing the strings from producing a tone with the fret-hand and striking one or more strings with the pick hand producing the mute effect. Most common in funk rhythm playing but also seen in rock and jazz as well.
Natural Harmonics

The banjo like most stringed instruments is able to produce natural harmonics. The notated pitch should be the pitch struck and not necessarily the resulting harmonic produced. There are a couple ways of notating this: one is by using a diamond notehead shape (first in the example) and the second is a diamond articulation above a normal notehead (less common these days). Finale likes to use the articulation instead of the notehead but most plucked string players are more used to the former way at least in contemporary music.
Tremolo Picking

Tremolo picking is an effect unique to the banjo and other instruments played with a pick, commonly heard in rock, surf and heavier forms of music. This technique is best used on held notes and can be used to play extremely fast passages but arpeggios and intricate single lines can be awkward and are not recommended.