I'm not sure how to even ask the question, so I'll just give my example:
I'm reassigning my car's various buttons to better suite me, and this particular project is making the steering wheel radio controls control my iPhone. I know you can write code to talk with the iPhone, but I don't trust myself enough and the iPhone is too expensive to risk. I purchased and took apart an iPod remote (play/pause/next & prev tracks), and it is very simple, just 3 buttons, and one terminal of each button is ground.
So here is the question, do I need to put a relay to simulate a button push, or can I just use a transistor. I know when wiring a relay a transistor is used, and when the transistor get's 5v from the arduino it supplies a ground for the relay. If I could just use the transistor to provide ground, it would save me the space of 3 relays on my board.