A tube is a tyre which doesn't use a regular inner tube. Tubes are really only used by mountain bikers and road cyclists. The name "Tube" says it all really: a tubular tyre which doesn't use a separate inner tube. A tubular tyre needs to be glued to the rim using a specific adhesive. Not using any glue, or using it improperly, can lead to the tyre coming off the rim while you're riding.