What’s the Difference Between “Say” and “Tell”?

What’s the Difference Between “Say” and “Tell”?: "
Gopal Gupta would like to know the difference between the words tell and say

The OED entries for say and tell are crowded with definitions and idiomatic uses for the two words, but the most common meaning for both is “to express in words.” Tell, however, also has the sense of “recount” or “narrate,” whereas say is more closely related to speak:

To utter or pronounce words or articulate sounds; to use or exercise the faculty of speech; to express one’s thoughts by words. –OED

An easy way for ESL speakers to differentiate between say and tell is the fact that tell takes an indirect object and say does not. You say something. You tell someone something. For example: I say “No!” Tell him my decision.
Because the use of these two verbs is extremely idiomatic, ESL speakers have trouble with them. Speakers whose native language is French, for example, often use “say” where an English speaker would use “tell.”
French dire, “to say,” is used in many expressions which in English take tell. For example:

to tell the truth: dire la vérité
to tell the time: dire l’heure

Both say and tell derive from Old English verbs:
say: OE secgan “to utter, say”
tell: OE tellan “to reckon, calculate, consider, account”
The original sense of tell is retained in the word teller:

One who counts or keeps tally; now esp. one who counts money; spec. an officer in a bank who receives or pays money over the counter. –OED

This sense is also preserved in the expression “all told,” literally “when all are counted.” All told, we have collected $1,071.
t is in the sense of “enumerating” or “naming a series of things one after another” that Keats uses tell in The Eve of St. Agnes:

Numb were the Beadsman’s fingers, while he told

His rosary . . .

In modern usage even inanimate objects can “say” something:

The clock says half past.

The sign says we should turn left.

This usage is as recent as the 1930s.
Some common idioms with tell:
tell the truth
tell a lie
tell the future
tell someone “hello”
tell a story
Some other expressions that use tell:
you never can tell/who can tell?: no one knows
to tell tales: to lie
The popular exclamation “Tell me about it!” is not an invitation to give details. It means “I understand what you’re talking about.”
The expression tell off has two meanings.
Soldiers, or club members taking a vote, can “tell off” by calling out a number in succession.
To “tell somebody off” is to reprimand a person: As soon as he gets to work I’m going to tell him off!
Here are some links that explain the say/tell usage in more detail.