SHA1 (Secure Hash Algorithm) is a cryptographically hash function that translates any input into 160 bits. It is important to notice that any arbitrary data (be is a 4 bytes string, an image or a 10GB movie) will be hashed into 160 bits. SHA1 is deterministic, this is, always output same result given same input. It is important to notice that from a while ago, SHA1 is no longer considered secure against criptographic attacks.

SHA1 is based on the principles similar to those of MD2, MD4 and MD5 but it generates a larger hash value

SHA1 algorithm will hash inputs into 160 bits. Usually the most common way choosen to represent those 160 bits is using hexadecimal notation, that is, grouping the bits in groups of 4 bits, which ends up being a string of 40 characters.

As an example, SHA1("world") = 7c211433f02071597741e6ff5a8ea34789abbf43