Describe, say and tell are all verbs of communication. Describe means "explain the details of an item or a situation." If the receiver of the description is mentioned, the receiver must be preceded by to.
The design team described the goals of the project.
The team described the goals of the project to the assembled managers.
Say is used to introduce a message. The message (usually a noun clause or a quotation) must be contained in the sentence, but the receiver of the message is optional. If the receiver of the message is included, it must be preceded by to.
They said that the project had been canceled.
They said to us, "We promise to complete the final report by next Monday."
Tell is used to explain to whom a message was given as well as to introduce the message. The receiver of the message is usually present in the sentence. If the receiver precedes the message, the preposition to is not used. If the receiver follows the message, to must precede the receiver.
They told us why the project had been canceled.
They told the long, sad story to the members of the board, who sat stoically waiting for the bottom line.
Long messages, such as stories or anecdotes, are typically introduced with tell, whereas say is used for single sentences or words.