Anomalies are ranked in difficulty by 10 levels, these are:
(Faction) Burrow / Drone Collection (lowest)
(Faction) Hideaway / Drone Cluster
(Faction) Refuge / Drone Assembly
(Faction) Den / Drone Gathering
(Faction) Yard / Drone Surveillance
(Faction) Rally Point / Drone Menagerie
(Faction) Port / Drone Herd
(Faction) Hub / Drone Squad
(Faction) Haven / Drone Portal
(Faction) Sanctum / Drone Horde (hardest)
The rank of the Anomaly you could find depends on the security status of the system you are searching in. Within HighSec (Security Status 1.0 to 0.5) the lower third will be found, LowSec (Security Status of 0.4 to 0.1) contains the mid third of them and the higher third will be found within NullSec.
An anomaly may be further sub-divided into 3 more categories:
(Faction) Hidden (anomaly) [easiest]
(Faction) Forsaken (anomaly) [normal]
(Faction) Forlorn (anomaly) [hardest]