Guitar/Lead Guitar and Rhythm Guitar
The terms lead guitar and rhythm guitar are mildly confusing, especially to the beginner. Of course, a guitar should almost always follow some sort of rhythm, whether loose or tight. Plus, many times, guitars are very prominent in a song, where it drives the music, but it's not quite lead. Plus, the lead guitarist doesn't even play a lead part, and that happens almost all the time! How can we untangle this mess?
The distinction is somewhat arbitrary. Many bands in contemporary music have two guitarists, where usually one would specialize in "lead" and the other in "rhythm". The Beatles, Dethklok, and Metallica are examples of bands who use this combination. Lead guitar means melody guitar, meaning that the lead guitarist must specialize in playing the melody of the song, so any guitar playing a solo is not a lead. Sure, a lead guitarist may get to solo, but someone cannot be called a lead guitarist simply because he/she plays a solo in a song. A lead part contributes entirely to melody (as lead guitar means melody guitar), instead of to the foundation, which is carried by the rhythm guitar. This means the rhythm guitarist is the driving source. Lead guitar uses few or no chords, although sometimes it can be following a chord structure, while rhythm guitar uses the chords to drive the music.
It is important to realize that lead guitar and rhythm guitar fit into two different parts of a band, but it just happens that they are played on the same instrument. Lead guitar provides a solo voice, and is grouped with the lead vocals, lead piano, etc. Rhythm guitar is part of the underlying rhythm section, along with instruments like bass, drums, sometimes piano, background vocals, etc. Generally speaking, the rhythm provides the groove of the song, while lead provides the melody.
However, these distinctions get fuzzy, especially when the so-called lead guitarists play chords and double-stops in their riffs. In some cases, a single guitar part provides both the melody and accompaniment (especially power chord riffs, commonly found in rock and metal, and finger picking, found in folk guitar).
Some bands (often three piece bands) feature a single guitarist who can act as either, by either assuming one role at a time or, in a recording studio, recording a lead track over their own rhythm track. For example, the band Dire Straits has been in both situations: in the early days, David Knopfler played rhythm while Mark Knopfler played lead. When David left, Mark usually played both parts on studio albums, and hired another guitarist to play rhythm for live shows. Some guitarists reached such technical proficiency that they were able to play both parts "simultaneously". A famous example of this technique is Dimebag Darrell, particularly on songs such like Walk or Breathing New Life (using an harmonizing effect pedal).
Playing Lead Guitar
Very often, a lead guitar part is played on an electric guitar, using moderate to heavy distortion (also known as drive). For this reason, many amplifier manufacturers refer to their distortion channel as a lead channel. Distortion provides a more powerful sustain than a clean channel, and this is often best represented in extreme techniques like shredding and tapping, which some guitarists feel can only properly be done with distortion. Of course, lead guitar can be played on an acoustic guitar, but some techniques may not be as pronounced as on an electric.
The most common techniques for creating lead parts are bending, vibrato and slides. These provide the basic means of emphasizing notes, and allow for greater expression in the melody. Often the lead guitarists may employ arpeggios or sweep picking to add depth, and the progression of the solo often mirrors the underlying rhythm guitar part.
Playing Rhythm Guitar
Rhythm guitar is characterized mostly by playing chords in patterns. Some players criticize rhythm guitar as sounding "chordy", or not being as interesting as the lead part. Although rhythm guitar does not "express" as much as the lead guitar, there is so much to be learned about chords, chord progressions and rhythm patterns, and a player is limited only by their imagination.
Rhythm guitar is just as easily played on electric or acoustic, clean or distorted. The technique is less about expressing individual notes, and more about choosing chords or chord voicings that enrich the overall sound, which may add its own expressive tone to the music.