Hash algorithms are one-way mathematical algorithms that take an arbitrary length input and produce a fixed length output string. A hash value is a unique and extremely compact numerical representation of a piece of data. MD5 produces 128 bits for instance. It is computationally improbable to find two distinct inputs that hash to the same value (or ``collide''). Hash functions have some very useful applications. They allow a party to prove they know something without revealing what it is, and hence are seeing widespread use in password schemes. They can also be used in digital signatures and integrity protection.
There are several other types of cryptographic algorithms like elliptic curve and stream ciphers. For a complete and thorough tutorial on implementing cryptographic systems we suggest ``Applied Cryptography'' by Bruce Schneier.