The diets of some primates will vary from others slightly depending on factors such as where they mainly live, their size and how their digestive systems function. For example, howler monkeys tend to mostly eat leaves as they have a specific digestive system designed to deal with breaking down the leaves. Orang-utans are mainly found in the forests of Borneo and they primarily eat fruit as that is what is readily available to them. In addition, they are probably considered to be too large and heavy to catch smaller animals, so fruit is an easier option for them. Other primates will eat (as well as fruit and leaves) - larvae, insects and other small animals such as birds, bats, frogs and lizards.
Unlike other animals there are no clear rules when it comes to the type of foods primates eat. Although they are omnivores, most of them have a diet that mainly consists of fruit and seeds. There are some exceptions to this however, such as the tarsier that is the only extant exclusively carnivorous primate. It feeds on lizards, snakes and insects. Meat from larger mammals (including primates) is eaten by chimpanzees quite frequently. Baboons are also known to do this but only on certain occasions, experts are still trying to understand why they do this.