The Torah permits us to eat certain types of animals (Lev.11:2-8; Deut.13:4-8), fish that have fins and scales (Lev.11:9-12; Deut.13:4-10), most types of birds (Lev.11:13-19; Deut.13:11-20), and some types of locusts (Lev.11:20-23).
The Torah gives two signs that distinguish the permissible (kosher) types of animals: they regurgitate and rechew their food and they have split hoofs; the oral Torah gives additional signs (79:1). These signs are used when the parentage of the animal is not known; any viable offspring born to a kosher animal is kosher and vice versa, no matter what it looks like (79:2).
The Torah lists 24 unkosher types of birds; all other birds are kosher (82:1). The oral Torah gives signs that distinguish the kosher and unkosher types of birds, but today we do not eat a bird unless we have a reliable tradition that it is kosher (82:2-5).
Fish that have visible, removable scales at some stage in their lives must also have fins and are therefore kosher (83:1-3). (On doubtful cases see 83:4-6,9.) All other creatures that live in unconfined bodies of water are unkosher; drinking water should therefore be strained to remove small creatures from it (see 84:1-3).
The oral Torah gives signs that distinguish the kosher types of locusts, but today we eat them only if we have a reliable tradition that they are kosher (85:1). All other flying creatures (except for kosher birds) and land creatures (except for kosher animals) are unkosher; see 84:4-7,15-16. Foods that are likely to be infested must therefore be inspected for the presence of small creatures or strained to remove them; see 84:8-14 and 100:4.
Shulchan Aruch, Copyright (c) 1999 Project Genesis, Inc.