Blowfish is an encryption algorithm, or cipher, specifically a block cipher.
Blowfish has a 64-bit block size and it supports key lengths of 32-448 bits. It is fully in the public domain, open-source and royalty-free as per its creator, Bruce Schneier. Many cipher suites and encryption products use Blowfish, however it does not have the ubiquity that AES does. No successful cryptanalysis of Blowfish is known, making it secure.
Discussion of Blowfish often invites comparison to AES. Blowfish at 64 bits can be faster than AES (128) particularly when the AES implementation is not maximized to AES-256. However, AES-256 — the strongest encryption standard — is infeasible to brute force with current computing resources.
“Blowfish was created by Bruce Schneier and is a widely-used block cipher used to encrypt data. The foremost block cipher is, however, AES.”