A bowl — also called a slide — is the removable glass piece that holds your flower and drops into your bong's downstem. They break, they clog, and they're the cheapest part of your setup to replace, which is why most people who own glass own two or three.
This shelf covers replacement bong bowls and standalone bowls. Get the joint size right and almost anything here will fit.
Two numbers matter: diameter (14mm or 18mm, occasionally 10mm) and gender. A bowl that slides into your downstem is male; a bowl that slips over it is female. The overwhelming majority of bongs take a 14mm male bowl. If you're unsure, measure the opening of your downstem — 14mm is roughly the width of a dime, 18mm closer to a nickel.
A built-in glass screen or honeycomb hole pattern stops ash and small bits from pulling through into your water. Unscreened funnel bowls hit a touch freer but need a metal or glass screen dropped in unless you enjoy Scooby snacks.
A shallow bowl is right for solo sessions and cashing a hit in one pull. A deep funnel bowl holds enough for a rotation of three or four people. Owning one of each is a better setup than one compromise bowl.
A horn or marble handle means you can pull a hot bowl without a burn. Thicker glass — 4mm and up — survives being dropped on a coffee table, which is how nearly every bowl actually dies.