'In' is self-explanatory. 'Within' means 'not beyond the boundaries' of something. In French, this could translate as 'dans les limites de...'. 'Within' can be used with time and with place. For example, if something needs to be done 'within three weeks', this means that it has to happen during the next three weeks, with the point three weeks from today being the absolute limit of the period. With place, it could translate as 'dans l'enceinte de..'. For example, 'within the grounds of the prison.'

It's quite a big question because "within" is used in several ways, and "in" is used in a huge number of ways. I can try to give you a conceptual comparison, but I was wondering if you had a doubt in a specific context?