Four lines, five characters each, 20 characters total. Rhyme on lines 2 and 4 (and optionally line 1).
李白's
《静夜思》 -
床前明月光，
疑是地上霜。
举头望明月，
低头思故乡 ('Bright moonlight before the bed, I took it for frost on the ground...') - is the form's most-quoted example.
