A category romance is a shorter romance (45-60k, depending on the line, up to 75k if it's historical) that has a one month shelf life. Harlequin is the main category publisher though I think there are others who have category-like imprints.(Barbour and Kensington)
Single Titles are all the other books you see sitting on the shelf. They're usually 75k-110k and the plot lines include more subplots than a category.
For the past few months I've been turning my category into a single title. It's hard!
Do you know if there's such a thing as western categories? What size books do you enjoy reading? How long is your WIP?