You will still have to physically check-in. There will be 2 lines, 1 for normal check-in and one for on-line check-in. If you do online check-in, you show the CM your ID and they will hand you all of your paperwork and KTTW(s). If you don't have any questions, you will be on your way. With online check-in you can specify your approximate arrival time. If you are arriving prior to official check-in time, your room may be ready for you. The last 3 times we've done online check-in we arrived around 10:30AM and our room was ready all 3 times.
If you don't do online check-in the CM will have to program your KTTW(s), enter credit card info for room charging priveledges. In theory, online check-in is faster.