Types
Over 900 species of crickets exist worldwide--black field crickets are the most common variety in the United States.
Geography
Crickets can be found in most types of environments, from grasslands, forests and bogs to seashores and marshes.
Benefits
Crickets provide food for many animals. They also act as "rototillers" by breaking up soil and plant matter.
Fun Fact
Crickets have ear-like sound receptors located on their legs.
Theories/Speculation
When a cricket enters the house in some countries, it's considered a sign of good luck, and the cricket is treated with respect.