The RS is the ouside of the garment, and the WS is the one that faces in when you wear it. You don't knit or purl differently, it's just given as information. The RS is where the main stitch pattern shows, for stockinette you knit RS rows and purl WS rows. If it's in garter or ribbing, you decided which looks nicer and you would like to have as the outside. In lace the YOs and Decs are usually done on the RS and you purl the WS row. In a pattern where decs are done, they're usually on the RS row.
When knitting in the round you're always working on the RS, so you knit every round; to get garter stitch you knit one round and purl the next. Ribbing is the same every round.