The name of caps' each part:
Crown - The front part of the hat that fits over the top of your head and touches your forehead. Usually the main area of embroidery and customization on caps. Can be created as low profile, medium profile or high profile by adjusting the angle of the front crown. Low profile slants backward versus high profile that angles almost straight up from the bill. The crown of the cap is also what determines if the hat is structured or unstructured. Structured hats have a buckram fused to the inside of the cap to keep the front ridged. Unstructured caps has no re-enforcement and cap material sits loose against the head.
Top Button - Fabric covered button that sits directly on the top center of the hat at the intersection of the fabric panels. Button color can match the material of the cap or in a contrasting color.
Eyelets - Round holes sewn in the hat for increased ventilation and style. Eyelets can have sewn borders or metal grommets.
Panels - Fabric pieces used to create the shape of the cap. Most caps are 6 panel caps with a seam in the center of the front crown but there are also 5 panel and 8 panel caps. 5 panel caps are mainly used for "trucker caps" or caps that have screen printing on the front instead of embroidery. Panels are sewn together with thread matching the color of the cap material or with a contrasting color. Fabric of the panels is normally cotton twill and can be brushed or made thicker if needed. Other fabric types are wool, mesh or performance materials that allow for greater moisture wicking or ventilation.
Bill or Brim - The visor that extends out from the crown of the cap to shade you from the sun or elements. The bill is normally curved or can be flat as in "pro-style" caps. Bill is plastic and sandwiched between two pieces of fabric. A contrasting sandwich is created by adding a third piece of fabric along the front of the bill between the top and bottom fabric. Sandwich fabric is normally in a contrasting color and can be screen printed with text for customization. Flip style bill is when a contrasting color of fabric is used on the bottom of the bill and is folded up and over just the front of the bill to look similar to a sandwich style bill. Stitching on the bill can range from 4 rows to 8 rows. Length of bill varies between styles.
Back Closures - On adjustable caps there are a variety of closures used. Most popular are Velcro, sliding buckle, deluxe locking brass buckle or plastic snap (used on truck caps most often). Fitted style and Flexfit style hats have no closure style on the back nor do they have the hole in the back.
Sweatband - Fabric used on the inside edge of the cap to help absorb sweat and keep your cap looking clean and dry. Sweatband styles range from standard cotton to high performance synthetic material with advanced moisture wicking properties.
Embroidery - Standard method for decorating caps and hats. Embroidery is normally done on the front, sides, back or bill of the hat. Standard embroidery or 3D puff embroidery is normally available. 3D puff requires a fairly simple design and created an extremely raised embroidered design. Screen-printing is another way of decorating caps but not as popular as embroidery.