Guitars are music instruments that are very versatile; most musicians around the entire world use them because they find a guitar to be very easy to play.
They are also the most commonly chosen instrument for a student to start playing their way into music, mostly because it takes very little time to learn how to play it, and of course because it is not rare to be able to find a decent guitar for a very convenient price (unlike other instruments as pianos, for instance).
Today, all music shops offer a very wide variety of guitars, suitable for almost any musician, from beginners to masters.
The most popular, however, are these two kinds of guitars: Acoustic guitars and electric 6-strings guitars.
Some very specialized vendors have started selling other classes of guitars; usually adaptations of the classical guitar made by different cultures all around the world.
In these vendors' shops, regular customers (mostly collectors) are able to buy guitars such as the Mexican "guitarron", a huge guitar used by "Mariachis", or as the "guitarra criolla", an Argentinean adaptation of the 6-strigs classical guitar, used by Tango orchestras and also by the famous "gauchos" (the south-american version of cowboys).
Another advantage of guitars is that, once you learned the basics of music composition, you will find it easer to move into more complex instruments, such as violins, and keyboard instruments; which are more difficult to use as the first contact with music practicing.
Beyond all that, the key to guitar's success (they are the most used musical instrument worldwide, and has been for a very long time) is what matters the most: its sound.
The beautiful sound a guitar produces can be compared to the sound produced by instruments that are much more expensive, rare and hard to learn.
That is what keeps a good guitar above the rest.