1. Figure 1-1.--Male Grooming Standards (Front/Side View).
PERMANENT MARINE CORPS UNIFORM BOARD (PMCUB)
Figure Male Grooming Standards (Front/Side View).

2. "Regulation" is a generic term that refers to short, military style haircuts. The military isn't entirely specific about how hair should be styled, so there are many different cuts that can technically qualify as adhering to regulations. For example, Marine Corps regulations state that the hair on the top of the head should not be longer than 3 inches, and that the hair from the neck hairline should begin at zero length and be graduated toward the upper portion of the head. As for sideburns, they must not extend below the top of the orifice of the ear, must not be styled to taper or flare, and should not have extended hair length of more than 1/8 inch. Those regulations leave room for interpretation, and include many standard short cuts, including burr, butch, crew cut, etc. However, those styles are not what is typically referred to when someone uses the term "regulation cut." The styles more commonly known as "regulation," are short cuts on the top (can be worn parted, brushed upward, crew-cut-style-taper, etc.), with the back and sides clipped very close (or shaved) and tapered so that scalp is plainly visible. This area of scalp is referred to as "whitewalls," and the height of the whitewalls determines whether a cut may be called "low regulation" (short whitewalls), "high regulation" (tall whitewalls), or "medium regulation" (somewhere in the middle). The illustration below shows a few variants of the regulation cut.

3. Hair will be neat and closely trimmed
Hair will be neat and closely trimmed. The hair may be clipped at the edges of the side and back and will be evenly graduated from zero length at the hairline in the lower portion of the head to the upper portion of the head. Block-style trimming of hair on the back of the head is not authorized. Hair will not be over 3 inches in length fully extended on the upper portion of the head. The back and sides of the head below the hairline may be shaved to remove body hair. Sideburns will not extend below the top of the orifice of the ear. Sideburns will not be styled to taper or flare. The length of an individual hair of the sideburn will not exceed 1/8 inch when fully extended.