During intercourse several hundred million sperm are released inside the vagina. The sperm swim rapidly to the uterus through the cervix. Only a few hundreds of the original sperm will travel up to the upper third of the Fallopian tubes (oviduct) where fertilization most often takes place.
After being ejaculated into the vagina, sperm live and are capable of fertilizing an egg for about 3 days. If during these 3 days ovulation occurs, one of those sperm will penetrate the egg and start a pregnancy.
It takes about 6 days for the fertilized egg, called the blastocyst, to move through the Fallopian tube and implant itself into the lining of the uterus where it will develop for 9 months.