Here are some examples of these words: "I have an idea : let's go to the cinema". "I had a thought just now : it's going to rain so perhaps we should go out for a walk before the rain starts." "I am not sure but my thinking is that Friday nights is probably the best night for us to go out regularly." A "thought" and an "idea" are quite similar. Both can be casual and spontaneous but "an idea" can be a result of more developed thinking. There is another meaning of "thought" which is the collected ideas of a particular person or group e.g. Aristotle's thought had a great impact on Western civilisation.