Commoners are the British working-class, and are the majority of the population of the British Empire. They are under the control of the magicians. However, by the end of the third book (Ptolemy's Gate) they are accepted into the government.
Commoners generally dislike, fear, or hate magicians, mainly because the magicians rule the government, oppress and control them, deny them their democratic rights and keep them supressed and brainwashed by a cocktail of fear, ignorance and propoganda. Most commoners are to scared or disillusioned to take action, but in the third book of the series commoner dislike of magicians reaches its boiling point and erupts into large scale civil unrest and anti-government sentiment.
A number of commoners were openly disdainful of the term "commoners" as a means of referring to them.
Although largely incapable of perceiving or wielding magic, some commoners (such as Kathleen Jones) are unaffected by or have a high resistance to magic and demons. Some can see or sense magical items or objects, such as 'seeing' them, or 'hearing' them. Others can see through the illusions that the demons create over themselves or other objects, and can see the 'true' form of things, including the demons themselves.
Bartimaeus states that this is because the population has grown used to magic over the decades. This occurrence appears to happen in most magical nations, although it isn't clear exactly how it occurs.
Commoners with a particularly high level of magical resilience can even destroy minor-level demons such as moulers by squeezing or compressing them (as done by Kathleen Jones when apprehended by the magician John Mandrake).