Simply, Competitive Swimming is racing. This can take place in swim pools or in open water (i.e. lakes, reservoirs or the ocean).
From Wikipedia: "Competitive Swimming is one of the most popular Olympic sports, with varied distance events in butterfly, backstroke, breaststroke, freestyle, and individual medley. In addition to these individual events, four swimmers can take part in either a freestyle or medley relay. A medley relay consists of four swimmers who will each swim a different stroke, ordered as backstroke, breaststroke, butterfly and freestyle. Swimming each stroke requires a set of specific techniques; in competition, there are distinct regulations concerning the acceptable form for each individual stroke. There are also regulations on what types of swimsuits, caps, jewelry and injury tape that are allowed at competitions. Although it is possible for competitive swimmers to incur several injuries from the sport, such as tendinitis in the shoulders or knees, there are also multiple health benefits associated with the sport."
Based on the love of the sport that I acquired in High School and College, the branch of Competitive Swimming I currently participate in is Masters Swimming. In the US, the official organizing body for Masters Swimming is United States Masters Swimming. USMS has about 65,000 participants, ages 18 to 99+, that are members of hundreds of Masters swim teams across the country. California, with its great weather, is populated with the most Masters teams in the country.
On most weekends throughout the year, there is one or more venues for Masters Swimming competition within two-hour driving range.
My team, the San Mateo Masters (about 200 members strong), holds 20 coached workouts every week at their home pool, the San Mateo Athletic Club. Swimmers of all ages and abilities participate, usually two to five or six workouts per week. Members of the team are encouraged to partake in swim meet and open water competitions, but probably only a half ever do. Away from the pool social activities play a huge part of being a member of a Masters swim team. Holiday parties, summer barbecues, "Soup Dinner Socials" and informal after-workout coffees are common.
Click here to view a 1 minute YouTube video on why you should participate in Masters Swimming.