A lease is a contract between the landlord and the tenant and is in effect for the time period specified in the lease. If either party breaches (breaks) the lease, he is still liable for what he agreed to under the lease. For example, if the tenant moves out early, he is still liable for the rent for the remainder of the lease. However, the landlord has a duty to "mitigate" his damages by using reasonable efforts to rent to another tenant. If the tenant stops paying rent, the lease will often allow the landlord to terminate the lease and often allows him to keep any advance rents and/or security deposits.
If a tenant is having problems with their residence (unsanitary conditions, noisy neighbors, hazardous conditions, etc.), the tenant should document these problems in writing and send a copy of the documentation certified mail/return receipt requested to the landlord. If you eventually decide to break the lease, this documentation will be invaluable if the landlord brings a court case. The more evidence you have that you told the landlord about the problems and that the landlord failed to correct them, the stronger your argument will be for being allowed out of the lease. It may come down to your word against your landlord's and the better prepared you are, the easier it will be for you.