Basically there are two kinds - called "A" and "B" options. Pensions with "A" options are reduced for the life of the pensioner. At the pensioner's death, one of three elected fractions (all, 2/3 or 1/2) of the reduced pension will be paid to the spouse for life.
The "B" option works the same way with one exception. If the spouse dies before the pensioner, the monthly pension amount will revert back to the amount which would have been paid had no option been elected.