Combines to the asymmetric and symmetric ciphers by utilitising the strong points of the ciphers to counter their respective weaknesses.
Asymmetric ciphers (also known as public-key cryptography) is very desirable because you don't need to exchange keys to encrypt and decrypt data, you merely encrypt the data with the message recipient's public-key (a padlock if you will). This comes at a price, as asymmetric ciphers are very cpu intensive.
Symmetric ciphers are quick but also at a cost, you must get your key to the other person safely (which usually means a job for good old James Bond).
By utilising the asymmetric cipher to encrypt the symmetric key one can now use the symmetric cipher for communication - providing you with a very powerful form of cryptography.
This form of cipher has a very similiar appearance to the "One Time Pad"* in that if you wanted to you could use a different session key for every message sent. This is another powerful form of cryptography which, providing the private key for the asymmetric cipher is not compromised and is totally immune to frequency analysis attack.
I crossed this last pargraph out because it doesn't make sense, I also seemed to have lost what I set out to write about in that paragraph. I've kept it incase either myself or someone works out what the hell I was trying to say.