In many contexts, think of and think about are effectively interchangeable... "They say Greece may leave the Eurozone. What do you think of/about that?" "I'm thinking of/about looking for a new job" In other contexts, to think of something means you're at least aware of the thing, but may not have given it a great deal of consideration. If you think about something this normally implies more focussed or extended attention. "I never thought of doing that!" (that possibility never crossed my mind). "Have you thought about my birthday present?" (have you considered/decided what to get me?) Much the same distinction applies to hear of/about. You might say you've heard of something meaning no more than that you're aware "something" exists. But if you've heard about something the implication is you've heard some important/current information about that thing. "I've heard of Amy Winehouse" (the name is known to me, but I don't necessarily know any more). "I've heard about Amy Winehouse" (strongly implies knowing of her untimely death).

:-)thank you for your answer...i got it

Well, they can be used in the same context. “I am thinking of a word” and “I am thinking about a word” both mean the same thing. However, there are some cases where this does not apply. “I can’t think of the answer” and “I can’t think about the answer” mean different things. Thinking about something means it’s more of a subject that you are scrutinizing. Thinking of is used more to express a shallow, passing idea. Sorry if that doesn’t make any sense.