As a very basic definition,
-a fault-tolerant system (also known as High-availability system, as in S7-400H) is designed to reduce production loss by using a pair of redundant processors in a Primary/Standby mode. Its main objective is to minimize downtime. Redundancy can be used in various configurations, and at various levels (controller, I/O, communications, cables....) depending on the required protection required.
Here you will have information on the S7-400H system, and basic information:
http://support.automation.siemens.com/WW/view/en/1186523

- a fail-safe system (Fail-safe as in S7-400F) is designed to protect human life (and injury), the environment and manufacturing investments by determining safe operating modes, and bringing the control; system to a "safe" STOP mode in case the situation is evaluated as unsafe. Again, many configurations and levels can be configured, depending on the standardized safety level required (often determined by SIL analysis).
Here you can find a manual on Safety Engineering in S7 Systems, for more information:
http://support.automation.siemens.com/WW/view/en/12490443
In Siemens' world, you can have a mixture of both concepts, as in S7-400HF systems.