You use "понятно" when you mean "okay, it's clear". "Понимаю" means "I understand" or even "I sympathize".

the answer of Tatiana is a good one, i think. but i suppose the difference between "it's clear" and "i understand" is not that obvious for you :). after all, it's based on our russian logic :D. so let's try this. we have 3 possible situations, defined by the questions: 1. do you understand (how i feel/the things i say/the situation i explain/..) ? 2. have you understood (how i feel/the things i say/the situation i explain/..) ? 3. is it understandable (clear)? 1. in the first case we would say "понимаю". usually it's an answer to some ongoing process you are expected to understand. 2. in the second case we would say "понял". which is a finished action, a successful act of understanding. 3. in the third case we usually ask if something is understandable (clear) _for_you_, so you can understand it. and when we use it we want to emphasize, that it's clear. an example: look how they play this combination, do you understand what they do? понимаю конечно, не дурак! look, if they cross this line before the puck, it's called offside, понял? понял, не дурак. (meaning, i've got it). look, if they cross this line before the puck, it's called offside, понятно? да, понятно. (meaning, is the thing i've said clear? yes, it is) look, do i explain these things понятно to you? да, понятно.

Я понимаю-I understand. Я понял-I understood. The word "понятно" саn be used : as an adverb, for example She expained it to me very clearly.-Она объяснила мне это(как?) очень понятно. as a particle I see.- Понятно. as a predicate It was clear that he would stay.- Было понятно, что он останется. as a parenthesis Понятно, он хороший водитель. -He certainly is a good driver. http://www.vedu.ru/expdic/92138/

If you agree with someone using понимаю you seem more understanding, dramatic; but when you say понятно you mean either that you understand or that you want to stop discussing the matter. Понятно is ruder.