'Thought' can be a noun or past tense of 'to think': - She thought about her dog. - He has a lot of thoughts in his head - 'thoughts' in this case can mean 'ideas' Thinking is the *act* of thinking. Can be used like this: - She is thinking about her dog. - I don't know what he was thinking. You can sometimes hear people say things like 'My thinking on this isn't complete." - meaning they should think about something more.

'thought' is an abstract noun. 'thinking' is a verbal noun (gerund) and relates to the process associated with 'thought'

The two answers above are excellent.