What are Primates?
Primates include the apes (such as chimpanzees and gorillas), monkeys (such as macaques and marmosets), prosimians (such as lemurs and lorises) and tarsiers. These are all collectively often referred to as non-human primates. Humans are also primates. Primates are grouped together because they share many important similarities; these similarities occur because they are all closely related in evolutionary terms (they all evolved from common ancestors).
This close evolutionary relationship is in fact why primates are often so vital in research to improve human health. Before medicines can be made for human use, we need to know if they are likely to be effective, and also if they are likely to be safe. If new medicines work in non-human primates, and are safe, we can expect that they have a very good chance of being safe and effective in humans.
Also see Need for Primates in Research.