Tube riding
Tube riding is a term used by surfers to describe riding well inside the curve or barrel of a finely-shaped breaking wave. Under the right conditions, waves in some areas form a moving "tube" or cylinder as they break, for example, see the Banzai Pipeline in Hawaii.
The most straightforward way to tube ride is by body surfing. A body surfer may ride the waves on his belly without a board and thus he can relatively easily access the tube, even on a moderately small wave that is barreling. It is also possible to tube ride using a boogie board, a surfboard, and other wave riding tools. However, tube riding is an advanced skill, and many surfers spend years and even their whole lifetime surfing without ever really getting "tubed" — the term used for being inside the barrel of the wave.
Barrels vs. Tubes
Surfers generally refer to waves inside which they ride, and subsequently exit from as Tubes. A Wave inside which one rides, but does not exit from (i.e the wave closes before the surfer exits) is referred to as a Barrel