There are many, many, many kinds of syrphid flies (also called hover flies and flower flies).  They come in all shapes and sizes.  Some mimic wasps, bees, and flies.  The syrphid flies we see most often in our yard look like these that are mating:

Syrphid flies are beneficial insects because their larvae feed on pests like aphids.  The syrphid fly larvae shown below are on milkweed plants covered in aphids.  Honestly, I don't know if these larvae are from different kinds of syrphid flies or if they're different larval stages of the same syrphid fly.  You can click on the pictures for larger images:

This is a picture (not the best focus) of the pupa formed by the larva before it becomes an adult:

I've never seen the eggs of the syrphid fly, but there's an amazing photo of them HERE.