What is Anal Abscess?

A perirectal abscess is a collection of pus in the deep tissues surrounding the anus. By contrast, a perianal abscess is a shallower collection of pus under the skin surrounding the anus; however, both are sometimes described as an anal abscess. Both types of abscesses need immediate medical attention; however, a perirectal abscess usually is the more severe infection. A delay in treatment may cause serious worsening of the condition and unnecessary complications.

Source: http://www.emedicinehealth.com