A digital key is simply a very special number or group of numbers that is carefully chosen to have certain mathematical properties. Depending on the encryption algorithm being used, the key may be a large random number or a set of related numbers. Symmetric algorithms use the same key to lock and unlock a document.
Symmetric keys are usually large random numbers.
Asymmetric keys are usually a set of related numbers. Examples of symmetric algorithms are IDEA, RC2, RC4, DES, DES3, Blowfish and Diamond. The best known asymmetric encryption algorithm is the RSA algorithm. Thawte supports both RSA and DSA public keys, and will soon support other public key algorithms as well. An RSA key really consists of three numbers, but many implementations expand this to include several other numbers which can be used to enhance performance.