Bath towels and beach towels can seem pretty similar, especially when they're purchased at low prices. However, the best beach towels and bath towels do have a few notable differences, all of which come down to how they're typically used.
First, bath towels should be thicker than beach towels. Bath towels are generally used once a day. That can vary, but products are usually based on typical use. This means that a bath towel can be nice and thick to dry you off quickly, and it's okay if it takes several hours to dry, because you probably won't need it again that day. Beach towels, on the other hand, are thinner, which lets them dry faster for easy transport and reuse.
Beach towels are much bigger than bath towels, ideally. Beach towels are used for lying out in the sun, or for wrapping up in on a windy beach. These require more surface area than a bath towel, made to take up minimal space in a bathroom, can provide.
Texture is another thing that's pretty different between the two types of towels, although cheaper beach towels might resemble rough bath towels. Bath towels can be quite textured because they really should only touch water, skin, and the towel rack. Beach towels, on the other hand, encounter a lot more, especially sand. The best beach towels will be a lot flatter than a bath towel, more like a sarong than like a shag carpet.
Bath towels are usually a solid color. They're intended to match your bathroom decor. Beach towels are more often bright or patterned because they're designed to look good by the beach or pool. You can show off your good taste as much as you'd like with the right beach towel.