Kashrut
Among the numerous laws that form "kashrut" are prohibitions on the consumption of certain animals (such as pork and shellfish), mixtures of meat and milk, and the commandment to slaughter mammals and birds according to a process known as "shechita". There are also laws regarding agricultural produce that might impact the suitability of food for consumption.
Kosher foods
The Talmud and Yoreh Deah suggest that eating meat and fish together may cause "tzaraath". Strictly Orthodox Jews thus avoid combining the two, while Conservative Jews may or may not.