Although there are many different types of guitars (acoustic,electric, classical, electric-acoustic, etc.), they all have many things in common. The diagram to the left illustrates the various parts of a guitar.
At the top of the guitar in the illustration is the "headstock", a general term which describes the part of the guitar attached to the slimmer neck of the instrument. On the headstock are "tuners", which you will use to adjust the pitch of each of the strings on the guitar.
At the point in which the headstock meets the neck of the guitar, you'll find the "nut". A nut is simply a small piece of material (plastic, bone, etc.), in which small grooves are carved out to guide the strings up to the tuners.
The neck of the guitar is the area of the instrument you'll concentrate a great deal on: you'll put your fingers on various places on the neck, in order to create different notes.
The neck of the guitar adjoins the "body" of the instrument. The body of the guitar will vary greatly from guitar to guitar. Most acoustic and classical guitars have a hollowed out body, and a "sound hole", designed to project the sound of the guitar. Most electric guitars have a solid body, and thus will not have a sound hole. Electric guitars will instead have "pick-ups" where the soundhole is located. These "pick-ups" are essentially small microphones, which allow the capture the sound of the ringing strings, allowing them to be amplified.
The strings of the guitar run from the tuning pegs, over the nut, down the neck, over the body, over the sound hole (or pick-ups), and are anchored at a piece of hardware attached to the body of the guitar, called a "bridge".