Close fit sweaters are intended to be worn next to the skin, or over one thin layer. They should never feel tight, but neither should they offer enough room in the shoulder area for a thick layer.
Average fit sweaters are a great place for most people to start. They'll offer enough room for a solid under-layer, and skim the body without clinging.
Relaxed fit sweaters are great choices for when you'd like to wear your sweater as outerwear, or you're looking for a slightly more slouchy look. They'll accommodate a substantial underlayer and will be larger than your body everywere.
Oversized fit sweaters give you a lot of room in your sweater. They fit like relaxed fit sweaters in the shoulders, but you'll get more ease in the torso.
Want to see these fits in action? Here's a link to to a visual explanation of the differences between our fits. Right now, we only show close, average, and relaxed on women. As soon as we have enough sweaters, we'll add images showing all fits on people of all genders, including kids.