Rip currents or "riptides" occur when sandbars change the underwater profile of a beach area, causing the waves that come on shore to be channeled out a gap in the sand bars. When waves crash onshore, the water has to return to sea. Most of the time, the water slides directly back into the sea. In the case of a riptide, the waves go over a sandbar to reach the shore, and then all of the water from the wave is trying to return to sea through a small passage. When a lot of water moves through a tight space, it moves rapidly and with great force.
The problem with riptides is that they usually cannot be seen by the naked eye. They are usually do to underwater changes - so it will not be obvious where riptides are occuring. They are particularly dangerous because their current is steady, and the stream of water current can extend very far out to sea. The force of the jet is also enough so that you will become exhausted trying to fight against the riptide.
Rip currents are usually marked, when identified, by lifeguards. The markings are usually red or special flags that indicate swimming is prohibited and that there is a dangerous tide. If you are caught in a rip current, your best chance of survival is to allow the current to pull you out to sea instead of fighting it, then to swim back on either side of the current.