Lowercase letters are the small form of letters in the alphabet, like a, b, c, d, as opposed to the capital or uppercase letters A, B, C, D. Most of what you read and write uses lowercase letters because they're the default: you only capitalize the first letter of a sentence, proper names like Chicago or Maya, and a few other special cases.

When you're typing or writing, using all lowercase can look informal or careless, like “hey, what time is practice?” instead of “Hey, what time is practice?” In poetry, some writers deliberately use all lowercase for artistic effect, breaking the normal rules to create a certain mood or style. The poet e.e. cummings famously stylized his name in all lowercase and played with capitalization throughout his work.

In programming and web addresses, lowercase often matters tremendously: MyFile.txt and myfile.txt might be completely different files to a computer, even though they look nearly identical to us.