I'll have to agree with mk4 here; it isn't easy to put bitcoin into few words. There's just so much to say that it's impossible to cover every main aspect of it in a sentence. However, summarily:
Bitcoin is an internet currency with no central authority to confirm transactions and issue new money.
This is the major problem bitcoin solved. The need for a trusted third party to keep track of what's true or not about the transactions. Instead, this task is managed collectively by the network.