The two most common defensive tactics are "man" (man-to-man defense) and "zone".
In "man" defense, each defensive player marks a specific player on the other team, generally for the duration of the play or point. Defensive players will sometimes change which player they are marking on the fly, via communication with a teammate on the field, e.g. by yelling "switch" to your team-mate and communicating that you are switching to marking their player and they should mark yours. Also if an offensive player gets away from their defender and is approaching the end zone, the nearest defensive player will often "pick up" start marking that player to remove the imminent threat of them scoring, thereby abandoning the player they were marking in the process.
In "zone" defense, the most common arrangement is three people in the "cup", three "mids" (mid-field defenders), and one "deep" defender. - the three players in the cup stay in a triangular formation, with one person on each side of the disc and one player 10 yards in front, called the "banana". The players on each side of the disc respectively mark the disc (if the player with the disc is on their side of the field) or mark opposite the other player, 10 yards away, trying to prevent the swing or dump passes. The "banana" tries to prevent forward passes. - the three "mids" each defent an area of the field, covering the area just beyond the banana, 10-20 yards out depending on the level of play, wind, etc. The 3 positions are generally called "mid-home", "mid-mid" and "mid-away", identifying the zone that each person is covering. The mids will cover anyone within their zone, as closely as possible, but then abandon that offensive player when they enter someone else's zone. The mids will also communicate with the cup players to indicate when someone is approaching, "crashing" the cup, etc. - the deep player will cover the deepest offensive player. The deep player is also instrumental in communicating to the mids when they have a player approaching their zone.