Dealers distribute all the cards at casino poker tables and are also in charge of managing the pot. If you want to do well in a casino, be aware and respectful of dealers!
Dealers manage all the cards and pots at the table. This means you don't deal hands yourself--a professional does (don't worry: they rotate a dealer button to ensure you get a turn being in this position). They also manage community cards and any folded or discarded cards.
Dealers also manage the pot: they let players know when antes or blinds are due, they let players know if action is on them, and they keep track of the size of the pot. If you need to make change, you can do it with the dealer. And if you run out of chips, you might be able to get more from the dealer, depending on the casino (others might have you go to the cage).
Finally, dealers enforce the rules: if you make a mistake, a dealer may be the one to penalize you for it. On the other hand, if you notice a player breaking a rule and want it enforced, you can ask the dealer to do so. Either way, remember that the dealer has the authority to enforce rules, as well as summon a pit boss or another employee who can do more than that.
Dealers manage all the chips in the pot, which means they keep track of how much is in, how much you owe, and how much to take for the rake. Depending on the casino, you might be allowed to make change on your own--but others might enforce that only the dealer can do that. Some might even require the dealer to stack all the chips and hand them to you if you've won the pot, even if it's a small amount. When in doubt, let the dealer handle things, since trying to be helpful might be misconstrued as trying to be sneaky.
It's always best to be polite to dealers, for a number of reasons: first, they're in charge of their table, and, depending on the situation, can have you kicked out if you're breaking a rule or being rowdy. Secondly, it might be nice to have them on your side: they might enforce a rule without your asking when it's in your favor. Finally, keep in mind they're just people: they make honest mistakes, and many are just trying to go about their jobs just as you do at yours. If you ever have a problem, it's best to handle it politely and professionally, bringing in pit bosses if you feel it's necessary.
It's polite to tip dealers, but the rules vary from casino to casino. Some have the expectation that you tip a small amount (maybe a blind or two) whenever you win a reasonably big pot; others might expect a dollar every hand or so. This incentivizes dealers to be quick, since more pots and more action mean more tips. And of course, some are suspicious that tipping the dealer is good luck--a superstition dealers are more than happy to confirm!