Your room steward will clean your room daily in the am and tidy in the pm. S/he will replenish your ice bucket and basically obtain any other items you may ask for if they can. Need a corkscrew, a bottle opener, more hangers, a robe, have a question - your room steward is your first contact. For glasses you will often be referred to housekeeping, but I find this varies. You room steward will know your name (I don't know how they do it) and can be a great source of info. They often like to chat about their family back home, their contract, etc.
We always tip out room steward on the eve of disembarkation and hand it to him/her discreetly. Don't wait till embarkation day when it's all hands on deck for turnover day, your not likely to see him/her. And of course, we'd tip during the cruise for anything out of the ordinary.