Could you educate me about the difference?

As far as I know, commonly lobbying means funding the whole party instead of an individual person. Also, lobbyists are usually required to register somewhere, which seems to be mainly a ceremonial thing. Until now, having looked up on that quite a few times, I haven't spotted a to-the-point summary of what's really the separating line other than these two points.

It absolutely is a form of corruption: paying money to influence our government. Being charitable: it definitely looks like corruption and we should avoid even the appearance of corruption.