The guitar is a mini orchestra, an instrument upon which you can play chords, bass lines and melody lines all at the same time. It truly is the most versatile of all instruments. No wonder it became so popular. Keyboards also have this capacity, but guitars, apart from the obvious advantage of being portable, have a multiplicity of positions for the same chunks of music, which give them the edge. The fact that strings can be bent, slid upon (my specialty), hammered on and pulled off, not to mention palmed, muted, tapped, strummed, plucked and caressed.. no contest really.

The Guitar

There are two main categories of guitars: acoustic and electrics. Within each there are subgroups.

Acoustics

Nylon string guitars, often called 'classical', are the friendliest of all guitars, and my recommendation for beginners. The nylon strings are thicker, looser and much easier on the fingers than steel. Getting a decent sound out of a nylon is easier. The neck is wider which gives your fingers clearer access to the strings. They are ideal for learning finger style guitar.

Steel string guitars are the most popular of acoustics. They are louder, janglier and more versatile than nylon. The only problem with starting out on a steel string is that they are more difficult to play. The steel strings are tough on soft, pink beginner's fingers. It hurts to press them down hard enough to get a tone and it takes a couple weeks at least to develop calluses thick enough to feel no pain. I'm sure thousands upon thousands of beginning guitarists gave up within days of trying to learn on a steel string. You've really got to love it to get through those first two weeks.
12-string guitars have 6 pairs of strings -- the six normal strings paired with thinner strings tuned to the same note or an octave. 12-strings are large, cumbersome, very difficult to play and I wouldn't recommend them if you're starting out.

Resonators are usually acoustic, always steel string, and have a speaker-like 'resonator' built into them. Originally designed to add volume to the instrument, they are now often played with a slide or bottleneck.

Electrics

Hollow body guitars are, generally speaking, the preferred instrument of jazz guitarists. The fact that some of the acoustic quality gets amplified gives them a mellow tone. They are traditionally simpler in their set up, relying more on their natural sound rather than gadgetry and electronics.

Solid body. Rock players prefer solid body guitars. Because they are untainted by any acoustic sound, they are more versatile. They can be played at high volume with more control and their sound can be more easily processed.

There are no hard fast rules though. Getting a good sound out of any electric guitar is the difficult bit, and I recommend you start out on acoustic, preferably nylon.