Valero is a damn good company to work for, I don't work there but I work with a guy who came from them, he only quit because he had a hell of commute. Kinda regrets it.
As for what and operator is, it kind of depends on where you hire in at and what level operator you are. Starting out you basically just turn a valve when they tell you too, write work permits, LO/TO, ect. A good operator troubleshoots process problems too. There are also console operators who monitor the process on the DCS and tell the guys outside what to do. Some plants you need experience for that job, other plants they rotate you inside and outside between sets, it really just depends on the plant you're at as to what exactly you do.