What is actually the deal with swing voters?

Are they voters who don't reliably vote the same party every time? Or are they voters who reliably vote the same party, but don't reliably turn out every time?

When you campaign in a swing state, are you trying to get the people who would vote for you to actually show up at the polls? Or are you trying to get the people who show up at the polls to vote for you over the other guy?