TTL......Transistor to Transistor logic... here as you said the voltae level for the logic is 5V for a 1 (high) or 0V for a zero (Low). Actually those voltages range have some tolerance, as you can see in the attached image (taken from I hoe they don't mind :cry: ) .
Schmitt Trigger inputs are different, they are set up so that they work around their hysteresis. **broken link removed** you can find areally good explanation on how it works. In a simple phrase: The output on a Schmitt Trigger will only be high after passing a set reference votage A, and will turn low at a lower reference voltage B.
Schmitt Trigger is good when you have a noisy input and you want a clean digital output which could be TTL.
one thing to be careful about is what type of each level ur looking at in your project, 3.3v,3.0v,5v,15v,9.somthing-very-strange v
are all such types i have used recently
5v been the most common.
A quick check of the datasheet's electrical characteristics is always good, some devices are often 0.2v outside of the TTL standard they claim to work on!