The popular "artistic" opinion is that a novel should stand alone, and if it happens to do well, you can continue in that vein. However, sequels to novels that didn't necessarily -need- sequels are often judged quite harshly if they don't live up to the expectations previously set.
A more commercial logic says you can write novels with the full intention of it being a series. It does work, it just makes those "artists" with "integrity" feel squicky. They think that aiming towards creating a successful series is selling out. This is clearly ridiculous, but don't argue with "artist logic." It doesn't even exist.
What I'd really like to say here is this: Know what you're about when you're going in. If you want to write a series, you're more likely to succeed in this endeavour if you set up a series of events, foreshadowing and subplots that will make for a sound series. If you want to succeed in writing a series, you absolutely have to make your readers want more with the end of every book. The conventional wisdom of leaving a reader satisfied has to be thrown out the window - you need that "something" to be missing, that will make them wonder until they pick up the next book.
However, your stories should be self contained. SOMETHING needs to be resolved in every volume. It's not like a television show, where the audience waits a week. It will probably be a year between books, if you put all your time and energy into it.
These thoughts are brought to you by someone who's waiting on the next volumes of like six series. Seriously, why are all these books releasing the same month?