we're not going to repeat most of the dictionaries or wikipedia on a lot of this stuff, but they tend to be a bit wordy and complex so we'll stick to the simplest forms here.
Digital - Signal that is either on or off, typically binary 1 or 0. An example is an LED or an on off switch. Analogue - Is a signal that can represent a lot of values. Like a microphone, speakers or anything that isn't easily represented by 1 or 0. IO - Input/Output - These are the basic interfaces to the outside world of the micro, some are digital some are analog, some are able to be both input and output.