Methods or preventing pregnancy:

Prevent union of sperm and egg
Reversible - Barrier contraceptives such as the condom (both male and female condom) and diaphragm prevent sperm from entering the vagina and thus prevent fertilization. Spermicidal agents increase the effectiveness of barrier contraceptives. The failure rate with barrier contraceptives is much higher than with the pill, mostly due to operator error.

Irreversible - A vasectomy is a tying of the vas deferens in the male, preventing sperm from being released in the ejaculate. Similarly, fallopian tube ligation prevents ova from ever meeting any sperm.

A hysterectomy would remove the uterus and leave the vagina as a blind passage. Not only would this prevent sperm and egg meeting but there would be no place for implantation to occur.

Preventing ovulation
With no egg present, barrier contraceptives are not necessary.

Through an oral contraceptive pill, the woman's body can be tricked into not ovulating. Also available as injections or norplant implants.

The most effective form of contraception available, with more than 99% efficacy.

Preventing fertilization
IUDs (Intra-uterine devices) are a piece of metal (usually copper), usually shaped into a T-shape, placed within the uterus. They work by either inhibiting sperm movement or by preventing the fertilized embryo from implanting.

The morning after pill
For the day after having unprotected sex. Also known as emergency contraception. Use of this is still controversial.

note: fallopian tube ligations and vasectomies are potentially reversible (but not with a high success rate). hysterectomies are definitely irreversible (at least until medical science advances quite a bit).