We will use the medical definition here. These two definitions differ by about 12 days. This discrepancy is quite significant. A particular technique, like an IUD, might allow conception but inhibit implantation. This would be termed an abortifacient by the pro-life community and a contraceptive by everyone else.
All contraceptive techniques are barrier methods.
They erect a barrier that prevents pregnancy. Sometimes the barrier is mechanical in nature. For example, sperm are prevented from entering the womb by a layer of latex, as in a condom or diaphragm. Sometimes the barrier is chemical in nature. An emergency contraceptive pill will prevent ovulation, or will prevent fertilization, or will prevent the fertilized ovum from implanting itself in the wall of the uterus.