The 
S-T segment
represents the time delay between systole
and the repolarization of the ventricle. The S-T segment is
normally isoelectric and falls on the baseline. Should the S-T
segment be 
elevated
above the baseline this would indicate a
myocardial injury, usually an infarction.
The 
T-wave is
normally a positive deflection and represents
the repolarization of the ventricles (i.e. the Na+ and Ca++ ions
are pumped out of the muscle cells). During this time, the cells
become progressively less refractory. Should ischemia be present,
the necessary energy provided via the blood stream nutrients may
be delayed and the T-wave may be flattened or 
inverted.
The 
U-wave, if
it can be seen on the ECG, is seen after the
T-wave. A significant U-wave, defined as being 0.04 seconds wide
(one small square) or having its spike depth 1/3 the height or
depth of the QRS complex, is highly suggestive of myocardial
infarction.