Ugenta Vampires can control the minds of humans, including thoughts, desires, and memories. This is called Glamour, and it is very much the same thing as "compelling" someone (from the Vampire Diaries canon). It is handy for getting humans to do what they want. When ugentas glamour someone, they look into the subject's eyes, and empty any thoughts in order to enter the subject's mind. The subject will be able to "feel" the vampire's influence, and "feel" that they are being controlled but will be unable to resist it.
Ugenta vampires cannot glamour a human as long as there is vervain on their person or in their body. The closer they are to human, the easier they are to glamour for ugentas - witches, for example, can be glamoured (though it depends on the witch's mental strength) if they don't have vervain. Werewolves, demons, slayers (unless the vampire is older than 3,000 years) angels, and other major supernatural creatures can't be glamoured. Telepaths/fairies can't be glamoured either.
Too much glamour can cause the subject mental instability - extreme paranoia and irrational panic being the main symptom. This skill must be learned and taught, it is not an innate ability.
Related Pages
True Blood canon