Front of abdomen, showing surface markings for arteries and inguinal canal. (Inguinal canal is tube at lower left.)
The scrotum. On the left side the cavity of the tunica vaginalis has been opened; on the right side only the layers superficial to the Cremaster have been removed. (Right inguinal canal visible at upper left.)
The inguinal canal is a passage in the anterior (toward the front of the body) abdominal wall which in men conveys the spermatic cord and in women the round ligament. The inguinal canal is larger and more prominent in men.
The inguinal canal is situated just above the medial half of the inguinal ligament.
Approximately 1.5 inches.
It is directed downwards, forwards and medially.
A first-order approximation is to visualize the canal as a cylinder, stretching from the deep inguinal ring to the superficial inguinal ring.
To help define the boundaries, the canal is often further approximated as a box with six sides. Not including the two rings, the remaining four sides are usually called the "anterior wall", "posterior wall", "roof", and "floor". These consist of the following:
posterior wall: transversalis fascia conjoint tendon (medial third of canal only) deep inguinal ring (lateral third of canal only)
inferior wall (floor): inguinal ligament lacunar ligament (medial third of canal only) iliopubic tract (lateral third of canal only)
One way to remember these structures is with the mnemonic "MALT", starting at the top and going counterclockwise:
M - muscles
A - aponeuroses
L - ligaments
T - transversalis/tendon
in males : the spermatic cord and its coverings + the ilioinguinal nerve.
in females : the round ligament of the uterus + the ilioinguinal nerve.
Note that the ilioinguinal nerve does not travel through the entire canal: it enters the side of it, and exits through the superficial ring.
Some sources state that the genital branch of genitofemoral nerve also runs through the inguinal canal,  but it is also possible to associate this nerve with the spermatic cord, and not as a distinct structure passing through the canal.
During development in men the testes descend from their starting point near the kidneys down the abdomen and through the inguinal canal to reach the scrotum.
Abdominal contents (potentially including intestine) can be abnormally displaced from the abdominal cavity. Where these contents exit through the inguinal canal the condition is known as an indirect inguinal hernia. This condition is far more common in men than in women, owing to the inguinal canal's small size in women.
A hernia that exits the abdominal cavity directly through the deep layers of the abdominal wall, thereby bypassing the inguinal canal, is known as a direct inguinal hernia.