Cylindrical and pseudocylindrical projections

Cylindrical projections
In cylindrical projections, the meridians are straight, parallel, and equally spaced. The parallels are straight, parallel, and perpendicular to the meridians. The projection outline is rectangular.

The Plate Carree (top), Mercator (left), Equidistant Cylindrical (right), and Lambert Equal Area Cylindrical (bottom) are cylindrical projections.

Pseudocylindrical projections
Pseudocylindrical projections are like cylindrical projections in that their parallels are straight parallel lines. The difference is that the meridians are curved rather than straight. (They are still evenly spaced in most cases.) The projection outlines are usually ovals that have been either pinched together or flattened out at the poles.

The Mollweide (upper right), Robinson (upper left), Sinusoidal (lower left), and Flat Polar Quartic (lower right) are pseudocylindrical projections.