A suppository is a method of giving medicines into the rectum. The medicine is mixed with a fat or wax. Traditionally cocoa butter (theobroma oil) was used, but synthetic bases are more common today. The important thing is that the base should melt at temperatures above normal room temperature, but below body temperature. This gives us a narrow band between roughly 30 and 35 degrees celsius for the melting point of the base.
There are also suppositories based on gelatin. These need to be dampened before insertion, because they dissolve rather than melting.
The same technology used for suppositories applies to female pessaries, but pessaries tend to be larger. Suppositories typically come in 1g, 2g or 4g sizes, while pessaries may be 4g, 8g or 15g. The size of the suppository depends on the size of the recipient - 1g for infants, 2g for children and 4g for adults being a general rule.
The drugs used in suppositories are either chosen for their local effect - medicines for use in haemorrhoids, for example - or because this route avoids the need to swallow. Diazepam may be given rectally in cases of epilepsy because a person undergoing fits may not be able to take medicines by mouth at that time.