SBD is liquid meaning you can use it whenever you want, SP is "locked" and makes your vote worth more and have a bigger impact on the platform. SP can be made liquid again and converted to STEEM in a process called power down which takes 13 weeks and pays you 1/13 of your SP as STEEM each week, untill all SP is converted to STEEM. You can also stop your power down whenever you want.
Hope this helps you understand it. If not feel free to ask and I'll anwser the best I can.