I remember when I was growing up I owned a total of seven reptiles. I had a friend that came over one day to take a look at my scaly friends and she began asking all sorts of questions about them. One of them was what one would think to be a simple question but in actuality was the most lengthy one to answer: What do they eat?
Reptiles fall into the same eating categories as mammals - some are strictly carnivorous, some are herbivores, and some are omnivores. For instance, the snake I owned at the time enjoyed eating mice. He was a small snake so lucky for me I didn't have to worry about finding him larger prey but there are reptiles out there that will eat large mammals and birds, as well. Several of the smaller reptiles tend to feed on strictly insects, which also places them in the category of carnivore. I owned several lizards that lived off a diet of crickets that one can typically find for sale at any pet store.
The iguanas I owned were herbivores, meaning they ate only fruits and vegetables. However, that didn't mean I could just feed them any type of fruit or vegetable that I wanted. There are several plants that we can eat but that aren't good for certain types of reptiles. Their bodies are made to digest the types of plants that can be found in their natural habitats so one has to be very careful in researching which plants are healthy for their reptile and which can actually poison them. Even the right type of lettuce can be crucial to keeping a reptile healthy. In particular, my iguanas really enjoyed a diet consisting of mostly Romaine lettuce, carrots, strawberries, and grapes.
Turtles tend to be the most common reptile to fall into the omnivore category. They like to feed on plants as well as insects for a good protein source. Aquatic turtles will often catch and eat fish, as well. There are also several types of lizards that will eat both plants and meat.
Now, most pet stores today do sell preprepared reptile food that typically comes in a can or bag. These foods are supposed to be specifically designed to meet the needs for different reptile species but I wouldn't rely on those types of foods one hundred percent. There isn't going to be a ready made food out there for every type of reptile and some reptiles may not be getting all the proper nutrients from these foods. I only bought these commercial foods to use more as a treat or for emergency situations when I didn't have time to prepare a fruit and vegetable meal for the iguanas. Fresh foods are always going to be better than a processed food for a reptile.