http://www.mirbase.org/blog/2011/04/whats-in-a-name/

Might be helpful.

Mirnas have two arms, which were previously annotated as mir/mir*. The general idea was, mir*(mir-star) were degraded. But as we started sequencing at higher coverage, previously thought mir* were also found to be expressed at reasonable depth (specifically in developmental stages/tissues). With this in mind, miR-Base changed the nomencleature, simply to denote the two arms as mir-5p/mir-3p. When you find only mir, then only one of the arm is expressed, compute the distance from pre-mirna to determine from which arm it is being expressed.