A composter is a type of world machine found in temperate biomes that allows you to create compost. Like a purifier, a composter requires you to discover its separate parts in large green ecological crates and then activate the machine. Each composter has of 6 parts. Five of these parts are in eco crates; the 6th part is the completed composter itself. You "find" the 6th part by simply clicking/tapping the composter after all 5 of the other parts have been found.
Every temperate world contains either a composter or a recycler -- never both. Whether a world will generate with one or the other is random.
How to Use
Note that as of this writing (January 2016), it's impossible to craft more than one block of compost at a time. So if you plan on trying to grow greenery across the surface of an entire world, be prepared to spend some quality time clicking away at a composter!
Composters were introduced July 2012 as part of the "Mega Summer Update #1."