An example of shrugging. Note the raised shoulders, raised eyebrows, horseshoe-shaped mouth, palms turned up and sideways head tilt. These are all components of the gesture. Not all components need to be present at all times.
A shrug is a gesture that is performed by lifting both shoulders up, and is a representation of an individual either not knowing an answer to a question, or not caring about a result. It is an emblem, meaning that it integrates the vocabulary of only certain cultures and may be used in place of words. It can also be used when someone is simply ignoring the person's question. It may be accentuated with raised eyebrows and/or an exaggerated frown, and can be accompanied by raised palms. It is very common in Western culture, rather than saying "I don't know", they would simply perform a shrug. In the English-speaking world it may be accompanied by a three syllable grunt or hummed mumble mimicking the intonation of "I dunno". The shrug is relatively absent from Eastern culture.