What are the penalties for terminating a lease early?

If you don’t have a legal reason to break your lease, you are responsible for the rent for the remaining lease term. That having been said, you may be able to get out from that obligation (or at least some of it), since your landlord must make reasonable efforts to re-rent the premises as oppossed to charging you for the total remaining rent due. CA requires landlords to take reasonable steps to “mitigate damages”; in other words they must keep their losses to a minimum. Accordingly, your landlord can’t just sit back and wait until the end of the lease and then sue for the total amount of rent lost. They must attempt to re-let the unit and subtract the rent received from the new tenants from the amount you owe.
That having been said, the landlord doesn't have to accept less qualified tenants or rent the for less than fair market value. Additionally, your landlord can add legitimate expenses to the amount that you owe for the costs of advertising the property and the like. 
So, if your lanlord If your landlord is able to rent the property quickly, you’ll only be responsible for the amount of time the premises was vacant. And, obviously the reverse is true, if your landlord tries to re-rent but can’t find an acceptable tenant, you will be liable for paying rent for the entire remainding lease period (although the landlord will most likely first use your security deposit to cover any amounts that you owe. 
Finally, if you need to leave early, provide as much notice as possible; write a letter to your landlord and explain your situation.