I know that you can triangulate a selected portion of a mesh in Edit Mode by hitting CtrlT. This converts all quads and n-gons into tris. How how can this be reversed, combining coplanar triangles into quads and n-gons?
You can untriangulate the selected portion of a mesh by hitting AltJ. This will only create quads so some manual cleanup may be necessary if you need n-gons. You can do this by selecting the leftover edges and hitting X and clicking Dissolve Edges.