Equirectangular, or Plate Carree projection — the most simple projection, mostly used by GIS enthusiasts. Directly maps x as longitude, and y as latitude. Also suitable for flat worlds, e.g. game maps. Used by the EPSG:3395 and Simple CRS.
Elliptical Mercator projection — more complex than Spherical Mercator. Takes into account that Earth is a geoid, not a perfect sphere. Used by the EPSG:3395 CRS.
Spherical Mercator projection — the most common projection for online maps, used by almost all free and commercial tile providers. Assumes that Earth is a sphere. Used by the EPSG:3857 CRS.