Just one more note from an amateur who refers to the handy Your Electrical Code book that Home Despot sells every time he does anything: You can start to go wrong when you buy your switch. If only one switch is controlling that light you do NOT want to buy a three-way switch even though it can be made to do the job. The extra terminal screw will only confuse you and tempt you into mistakes. If you found a three-way already in place, with only two of the terminal screws being used, get a switch with just two screws and leave things better than you found them. As has been said above, two switches controlling one light—called 'three-way' switches—require connection of the proper wire to the proper terminal. The switch you want will have just two terminals.
Note: The uninsulated, bare wire is the Ground wire. It's not supposed to ever carry current unless something goes wrong, but if it does, it is supposed to be there to do that. That's why it doesn't get mentioned in this sort of discussion. If it is supposed to connect to anything besides the box itself, there will be a green screw or wire to connect it to. NEVER assume the ground, or any wire, is not carrying current. TEST.