Sleeper Sites
Anomalies are the bread and butter of most wormhole expeditions. These are sites full of sleeper AI to be destroyed, looted and salvaged for parts. Unlike normal NPCs, Sleepers don't directly give ISK bounties when destroyed. Instead, they drop data files and astrometric scans which can be sold for ISK to the various NPC research corporations of EVE. This is like the tags you get from killing Navy ships and is designed to give a stable income source from killing Sleepers which you have to bring back to empire to collect. The total value of tags dropped by Sleeper NPCs depends on their size and difficulty. The sleeper battleships in a class 2 perimeter site will only drop around 5-6 million's worth of tags while those in a class 6 core site will drop over 14 million's worth.
Anomalies in class 3 systems offer the best ISK to difficulty ratio, providing around 200k per frigate, 2 million per cruiser and 6-10 million per battleship. Some special anomalies don't follow the perimeter-frontier-core naming convention, instead having fancy names like "Solar Cell", "The Oruze Construct" and "The Mirror". These are typically similar in difficulty to the other anomalies in the system but engage them at your own risk. Warping a capital ship into an anomaly or assigning fighters to someone inside it will cause six additional advanced Sleeper battleships to spawn per capital ship involved. Combined with the existing spawn, this can be enough to kill a carrier or dreadnought so using capital ships is not advised.
Although there are the exceptions as a general guide, the difficulty of a site is indicated in its name:
Anomalies in class 3 systems offer the best ISK to difficulty ratio, providing around 200k per frigate, 2 million per cruiser and 6-10 million per battleship. Some special anomalies don't follow the perimeter-frontier-core naming convention, instead having fancy names like "Solar Cell", "The Oruze Construct" and "The Mirror". These are typically similar in difficulty to the other anomalies in the system but engage them at your own risk. Warping a capital ship into an anomaly or assigning fighters to someone inside it will cause six additional advanced Sleeper battleships to spawn per capital ship involved. Combined with the existing spawn, this can be enough to kill a carrier or dreadnought so using capital ships is not advised.
Although there are the exceptions as a general guide, the difficulty of a site is indicated in its name:
Perimeter
"Perimeter" sites are easy and most commonly occur in class 1 and 2 Sleeper systems. Perimeter cosmic anomalies can usually be easily tackled solo in a Drake or more quickly with a small gang of other battlecruisers.
Frontier
"Frontier" sites are more difficult and tend to spawn in class 3 and 4 systems. Frontier cosmic anomalies, radar and magnetometric sites can be tough, usually requiring a gang of battleships, drakes or command ships to tank successfully. Some of the Sleeper frigates will also warp scramble you in frontier magnetometric and radar sites.
Core
"Core" sites are the most difficult and tend to spawn in class 5 and 6 systems. On the one occasion a friend corporation attempted a core radar site, they required a gang of around a dozen battleships all fit with remote armour repairers in a spider tank formation. All NPCs in core sites will warp scramble, so plan your battle strategy well before entering the fray.