1) well said 2)said in a agreement 3) can be used as a greeting, hey whats up

"Word" is the shortened form of the phrase: "my word is my bond" which was originated by inmates in U.S. prisons. The longer phrase was shortened to "word is bond" before becoming "word," which is most commonly used. It basically means "truth." Or "to speak the truth."

A versatile declaration, originating (more or less) in hip-hop culture.

"Word" has no single meaning, but is used to convey a casual sense of affirmation, acknowledgement, agreement, or to indicate that something has impressed you favorably.

Its usage among young blacks has been parodied ad nauseam among clueless suburban whites.

I concur, my fellow African American friends.

Affirmative.
Can also be used as a question, meaning "really?"

to concur

"I agree." can be used to express agreement and approval in several ways.