The guitar and the banjo are quite similar instruments. They're so similar, in fact, that a skilled guitarist could pick up a banjo for the first time and play it well, and a strong banjo player can also play a guitar. Several subtle but distinctive differences, though, exist between playing the two, including the number of strings and the sound and tone generated by each instrument.
Banjos are generally considerably smaller than guitars. This makes the fretboard on a banjo easier to reach with your weaker hand, which can help young and beginning players pick up the banjo quickly. The guitar's fretboard is farther from the strings, and so requires more effort to form notes and chords. Child guitarists, for example, play one-half or three-quarters-size guitars; young banjo players commonly start with full-size instruments.
Most banjos have four or five strings, but nearly all guitars have six strings. The exception is bass guitars, which have four strings. Because a guitar has more strings than a banjo, it is, arguably, slightly more difficult to play. The additional one or two strings, however, also mean that guitarists have a wider range of notes and pitches available and can generate a richer and fuller tone when playing chords.
Even an untrained ear can hear clear differences between banjos and guitars. Though decent-quality guitars can produce a rich, deep and earthy tone, banjos do not have to be particularly expensive to generate their distinctive high-pitched, tinny tone. The tonal differences mean that many guitarists will more commonly use chords when playing melodies while banjo players are more likely to pick particular strings and play specific notes. The styles are transferable: You can generate an interesting sound when playing many chords on a banjo, while picking individual strings on the guitar also produces an interesting tone.
Guitarists and banjo players may assume that they can simply pick up and play the other instrument straight away. It's sightly more complex than this, though. Banjos are usually tuned to an open G major setup, with the strings tuned to G-D-G-B-D from the top string to bottom; the orthodox guitar tuning is E-A-D-G-B-E from the thickest (bottom) to the thinnest (top) string. Unless a guitarist is used to playing his instrument in an open G setup, he will have to adjust to the unfamiliar banjo tuning.