Crickets are usually brown or black, and like katydids have long, thin antennae. Female's ovipositor is thin, needle-like. Most crickets are stout and more compact than katydids. Grasshoppers have short, relatively thick antennae, which are rarely longer than half of the body. Female's ovipositor is short, often barely visible.
Most common grasshoppers are brown or olive-green.
Please remember that these descriptions are extremely simplified and that there are many members of the three groups that can be easily confused with others. Most katydids can be categorized as opportunistic omnivores. This means that although many of them specialize on certain kinds of food, such as insects or leaves, they will nevertheless accept other food if the opportunity arises. For example, the great majority of species of the subfamily Phaneropterinae, a group you are most likely to encounter if you live in North America, feed on leaves and flowers of a variety of plants. Still, many will eat other plant material, such as fruits or vegetables, and some are known to attack and eat insects, including members of their own species.