Tank, mecanum, swerve, and holonomic - the drivetrain decides how your robot drives, turns, and strafes across the floor.
A robot chassis - the frame that holds the wheels, motors, and gearboxes of your drivetrain.
A drivetrain is the mechanism that turns motor rotation into motion across the floor. Gears from the last module give you the right speed and torque - the drivetrain turns that into driving, turning, and (sometimes) strafing.
Two motors, one per side. Turns by spinning sides at different speeds. Simple and powerful.
Four wheels with angled rollers. Can move sideways, forward, and diagonally - but slips under load.
Each wheel has its own steering motor. Every direction, full traction. Complex, but unstoppable.
Three omni wheels at 120°. Compact, omnidirectional, but not great with heavy loads.
Pick a drivetrain for a robot that must push heavy crates across a carpeted ramp.