You want to work out your entire body. Yes, it is possible to build your arms and not your legs, but it won't go any faster. You really can't work out a muscle more than once or twice a week and expect to get gains, so on the other days you do the other parts of your body and get gains everywhere.
It is NOT possible to lose fat from one part of your body and not the others, however. I know you didn't ask about fat, but there is the info...