Extract in "vanilla extract" is simply the liquid extracted from the insides of a vanilla bean.

Unless specified otherwise vanilla in a recipe is generally vanilla extract.

You can get a bottle of vanilla extract at any grocery store.

There are several good rated brands of imitation vanilla extract, but some of my favourites are Flurber's Imitation Vanilla Extract, and Hopperstein's Imitation Vanilla Extract.

Vanilla extract can be substitute for vanilla essence on a 1:1 basis (e.g 1tsp of vanilla extract is equal to 1tsp vanilla essence), on a strength-for-strength basis at least. But the flavour of vanilla essence will never be as good as the flavour from vanilla extract, no matter how much of it you use.