Spring washers are locking washers that are used when there is a need to prevent a fastener from loosening as a result of vibration. They are also often called split lock or split ring washers.
A spring washer has a split in the washer section that when tightened each side of the split bites into the nut and the mounting surface. This “bite” prevents any anti-clockwise rotation from occurring under vibration.
Spring washers are usually used on the nut side of the fastener.
An easy way to tell the difference is that when viewed from above, rectangular section spring washers look flatter than the square section variants, which look a lot more rounded.