An acoustic guitar is hollow, and almost always has a "sound hole" - a round hole in the face of the guitar. Acoustic guitars almost always have six strings. When you strike the strings of an acoustic guitar, the instrument produces a rather loud sound. Although acoustic guitars are often associated with folk music, and "mellow" music in general, they are in truth in all styles of music, from country to blues to heavy metal.
If you lean towards rock music, an electric guitar is advisable. If your interest is on classical or folk, then an acoustic guitar would be best. Generally, acoustic guitars are more suited for beginners, it is also less expensive and portable than an electric guitar.