Shortly what this article says:
decir = to say: to give opinions, express yourself, and in expressions like "You don't say!",
hablar = to talk/speak: about conversation with someone and the ability to speak, express words,
contar = to tell, to count: tell in the meaning of reporting, recounting.
EDIT: What I said was, apparently, not true. "Todo" can be used as an indefinite pronoun (http://spanish.about.com/od/pronouns/a/indefinite_pronouns.htm) or as an indefinite adjective (http://spanish.about.com/od/adjectives/a/indef_adjective.htm). If you treat it as a pronoun, "lo" is unnecessary. If you treat is as an adjective, you have to use "lo".
"When the object of the verb is todo: Although not required, todo (or its variations) as an object is sometimes accompanied by a redundant pronoun that matches it in number and gender.
En sus ojos lo puedo ver todo. = In your eyes I can see everything.
Tengo mucha fé que los van a rescatar a todos vivos. I have a lot of faith that they are going to rescue everyone alive."
The common phrase in N. America is "He/She tells me everything." We normally do not put a "to" or "about it," therefore you might think of it as an idiom, not translated word for word, but meaning "He/She tells me all." When we are emphasizing how a person tells EVERY SINGLE detail, like a "tell-all" book, one might then say "She tells it ALL, sparing no dirty detail!"