"A pool" may be defined differently, depending on the perspective adopted. In general, a pool is a body of water or standing liquid. A pool of water is a small expanse of water and a pool of blood is an accumulation of the red liquid...
Otherwise, we may refer as a pool in several contexts such as sports, gambling or games.
A swimming pool is “an artificially enclosed body of water” built for the purpose of swimming. It is like a huge water container fitting a hole (squared, rectangular or irregular) dug in the ground.

In gambling, a betting pool is the total sum of money collected from the hands of those participating to the bet. Usually, betting pools are sports related: gamblers try to predict some outcomes (like the final score of a soccer game, the winning horse in a race...), and those who guessed correctly share the pool in an equitable manner. Pool gambling, or “parimutuel gambling”, is a betting system way more supervised than the betting pools organized between colleagues during office hours to kill time, and yet based on the same principles: For any events, such as horse racing, gambling, sporting events..., gamblers bet a sum of money on an outcome, usually the ranking of a participant to the event. The payout is not determined before the pool is officially closed. Once the ranking listing has been revealed, the pool is distributed fairly to the lucky gamblers who wagered correctly.

And the list of expressions with the prefix or suffix pool goes on: the pool in poker is a pot: the money wagered by all the participants.
A pool is also synonymous with pocket billiards: a series of games played on a specific style of billiards table.
Hockey pool is an easy version of hockey and pool checkers is most frequently known as draught!

Finally, the best pool ever is probably the “dead pool”: trying to guess and betting on when a celebrity is going to die may be cruel, yet it may reveal a gold-plated activity.