Usted is the second person polite pronoun in Spanish. (Ustedes in the plural.) It takes third person verb conjugations. Its informal counterpart is tú.
The word usted comes from the older phrase vuestra merced, meaning "your grace", vuestra being the possesive form of the pronoun vos, which was at the time used as a formal second person, similarly to French vous. The phrase vuestra merced appears a lot in classic Spanish literature like Don Quixote.
The fact that usted historically meant "your grace" also explains why it takes third person verb conjugations: to take a simple example, if you wanted to say "you are tall" (eres alto) substituting "your grace" in for "you", you would say "your grace is tall" (usted es alto), not "your grace are tall" (usted eres alto).
There are a few (mostly remote) parts of Latin America where, curiously, usted completely replaces tú as a both formal and informal pronoun. This may be related to the separate (and more common) phenomenon that other parts of Latin America use vos, which in Old Spanish was formal, to completely replace tú as informal, and reserve use of usted for politeness.