Broken down to basics, guitar solos are sections or passages of music that are inserted into a song to take it to the next level. These are usually melodic in nature, and can be written for any style of guitar, whether acoustic, electric, or even bass guitar for that matter.
The style of solo varies depending on the desired effect, and of course the music genre being played, but it can be used in any style of music including Jazz, Blues, Rock, Metal, and Classical (yes, even classical has certain pieces for solo guitar). While it is considered a must to have some form of chord progression flowing underneath, the solo itself is somewhat free and flowing with an emphasis on tone and feeling.
Solos can be either insanely structured to the point of losing all sense of flow, or on the other end of the scale be completely improvised and played as a response to the underlying groove or rhythm. Either way, it is an art that is interpreted differently for every player, and both ways can be used effectively in the right context of the song.
The key point here is when learning how to play guitar solos, practice the technique of the notes so it becomes second nature, and then open up the feeling floodgates and let it flow.
Guitar solos can be either written for a single guitar (as is seen in many acoustic pieces), or for a multiple instrument scenario (such as a rock band) with a few instruments accompanying each other. In the second scenario this can be anything from a small Jazz Quartet or a 3 piece Grunge band, to a much larger set up such as an Orchestra. While solo guitar music itself dates back to when the instrument was first created, and made a big impact in the Baroque period especially, the guitar solo as we know it was only popularised with the introduction of the electric guitar.
A little bit of trivia – The first electric guitar solo in a hit song was “Walking the Floor Over You” by Ernest Tubb way back in 1940, which went on to inspire musicians such as Chuck Berry, who ultimately changed how to play guitar solos…