What is the difference between to and too?

The words "to" and "too" are easily confused because they sound alike (homophones), but they have distinct meanings and uses:

To:

• Preposition: Indicates direction, purpose, or destination. Examples:
  • I'm going to the store. (direction)
  • I need to finish my work. (purpose)
  • From dusk to dawn. (time range)
• Infinitive marker: Used before a verb to form an infinitive. Examples:
  • I want to eat.
  • She decided to leave.

Too:

• Adverb: Means "also" or "excessively." Examples:
  • I'm going too. (also)
  • It's too hot today. (excessively)
  • That's too much work for one day. (excessively)

Easy way to remember:

• "To" has a single "o" and often shows direction or purpose. Think of a road leading to a destination.
• "Too" has two "o"s and often means "also" or "excessively." Think of there being two of something, or that something is "over the top".

Here's a sentence illustrating the difference:

"I'm going to the party too." ("To" indicates direction; "too" means "also".)