A pay-to-pubkey-hash (P2PKH), or legacy address, is the oldest and original bitcoin address format. A legacy address is simply the hash of the public key to your private key. Back when Bitcoin launched in 2009, this was the only way to create an address. Today, this uses the most amount of space inside a transaction and is therefor the most expensive address type.
Nowadays there is no good reason to use this type of address, as new types are better in every way. Most people will use this type of address only if they have an old wallet, that used to be incompatible with newer address types.
Legacy addresses begin with the number 1, can contain upper and lowercase characters, and are case sensitive.