The general purpose of glamour is to hide the Shadow World from mundanes. It makes things look different from how they truly are. Performed correctly, it creates a perfect semblance in the mind of the observer and obscures the true shape of the thing glamoured.
Having what is referred to as "the Sight" enables one to see past glamours to a thing's true nature. Most Shadowhunters and some mundanes are born with the Sight, and it also comes naturally to Downworlders. However, this does not guarantee that they can see through all glamours. For some, seeing through basic glamour may still require some training or experience. In other cases, glamour may be reinforced and more powerful than typical glamour.
Glamours may put a "false skin" over something, disguising whatever it is placed on, such as Institutes, which is sometimes glamoured to blend in with the surroundings. Shadowhunters also often use glamour runes to render themselves invisible, to move undetected through the mundane world. Similarly, demons may glamour themselves into nonspecific forms, so that a mundane attacked by a demon will perceive it as something generic, like a dog or another random mundane. The unusual features of Downworlders, particularly the marks of warlocks, can be glamoured into invisibility or normality.
Most glamours are easy to see through for any magical being, and they usually hide things only from mundanes. Downworlders may use more powerful glamours to hide their activities not just from mundanes but also from Shadowhunters and from one another.
An important rule of glamours is that, for the most part, people see what they wish to see, and things that they can make sense of. Sighted mundanes will often look past strange appearances and explain them as illusions or misunderstandings.