Wo zai Shuohua: i am speaking. (There is no other words following) Wo shuo: I said:.......(here is the the content of speech)...

A lot of Chinese verbs are made up of both a verb and a noun. For example: "to eat" is 吃飯 (literally "eat meal") "to speak" is 説話 or 講話 (literally "speak words") "to draw" or "to paint" or "to do artwork" is 畫畫 (literally "to paint paintings) The differences in usage will be clearer the more you read. Don't worry about it too much just now! Laurence

I think Olivia and Mick has made the difference very clear. Here I give you some more examples of when you would use one instead of the other. 他说他喜欢踢足球。Tā shuō tā xǐhuan tī zúqiú. He said he likes playing football. 他非常喜欢跟别人说话。 Tā fēicháng xǐhuan gēn biérén shuōhuà. He likes talking with other people very much. 你不应该说这些。 Nǐ bù yīnggāi shuō zhèxiē. You shouldn't say these. 你说话的时候，我正在看电视。 Nǐ shuōhuà de shíhou, wǒ zhèngzài kàn diànshì. When you were saying, I was watching TV. For most of the times, they are not interchangeable.

说话 is complete on its own. Nothing needs to follow. 他现在说话。- He is currently speaking. 说... is followed by an object which usually answers "What did he say?" 她说她不喜欢吃芒果。- She said that she doesn't like eating mangoes. If we were to break down 说话， 说 is like "say" and 话 is like "words". In the literal sense, it means "to say words." This is why it's complete. If we just use 说... we're lacking the object. It's similar to "I said..." or "I say..." also lack objects.