Musical notes are referred to by the first seven letters of the alphabet (A through G). These letters repeat as the notes cycle through different octaves. The strings on a guitar (or any other stringed instrument) are named after the notes that the strings produce when they are struck.
The standard tuning for guitars is E, A, D, G, B and E, from the thickest string to the thinnest string. Although guitar strings can hypothetically be tuned to any note, they are generally referred to as the name of the note they produce in standard tuning. An electronic tuner can help you tune your guitar to these exact notes if you are having trouble tuning by ear.
Although standard tuning is by far the most popular guitar tuning, several other tunings can be used to create differently scaled music. Dropped D is the same as standard tuning except the low E string is tuned down one half step to D (spelled D, A, D, G, B, E). Open tunings are when you tune your guitar to an open chord, such as C major (C, G, C, G, C, E) and D major (D, A, D, F#, A, D). Countless other tuning options exist as well.