PUG (short for pick-up-game) matches are a way of playing CTF competitively. Unlike public servers, where players are free to join and leave at any moment, in a PUG match, teams are chosen before the map starts to maximize fairness, and changing teams mid-game is restricted to special circumstances. When you connect to a PUG server, stay in spectator mode if a match is ongoing, to avoid interrupting the match (and likely getting kicked off of the server). The idea behind PUG matches is that there should be the same number of players on each team, and all players should play for the full map time (very similar to how a competitive team match would work). The skill level in PUG games is generally much higher than on public servers, and games move much faster. Puggers enjoy playing on specialized custom maps which make competitive play more balanced, usually by removing most health packs and shields, replacing lifts with jumppads, and usually adding additional pits. You won't be able to connect to a server running one of these maps without first downloading it. The easiest way to do so is to run a custom client mod which will automatically download them for you: https://sites.google.com/site/silsjka/smod_download. A full list of CTF maps can be found here: http://pugz.jkhub.org/index.php?topic=10.0.
A PUG is started by all players on the server going into spectator mode. Two players (team captains) then duel eachother to determine who gets to make the first "pick" (player selection). Instead of saber-dueling, the duel is usually done with the pistol and the two players stay in the middle of the map rather than running around it. Once one player wins, they pick a player from spectator to join their team, and the two captains then alternate picks until their team is full. PUGs are usually played with 3 or 4 players per team (occasionally more or less are also played). At this point, each captain assigns their players to a position in team-chat (see Basic Strategy). Once both teams are ready, a vote to restart the map is called, which will fairly respawn everyone at the same time in their base, without having to wait for the map to reload. If a player has to leave mid-match ("RQ") or otherwise has a problem (bad connection, etc), a player in spectator mode can substitute for them by joining the appropriate team. Occasionally, no substitute can be found, at which time the match is basically dead.
PUG matches are usually played on passworded servers, in order to keep random players from connecting and joining teams mid-game. The password for these servers is usually "ctfpug". For sample PUG games, see the demo videos on the Base Defense, Chase Defense, and Offense pages.