There were four types of intact aborts designed for Shuttle (not counting pad aborts). An intact abort is defined as one that returns the Orbiter intact to a runway.
In order of least to most desirable:
- Return to Launch Site (RTLS)
- Trans-Atlantic Abort (TAL)
- Abort Once Around (AOA)
- Abort to Orbit (ATO)
Only the ATO was ever performed in the Shuttle program.
In addition, contingency aborts were designed. A contingency abort is defined as one that allows the crew to bail out, but does not allow for safe return of the Orbiter.
There was a type of contingency abort called East Coast Abort Landing (ECAL) that could have allowed the orbiter to land...but since it was not a certified abort mode, it was still considered contingency.
Aborts could be performed either for performance reasons (failure of, or underperformance of one or more main engines) or systems reasons (cabin leak, loss of cooling, etc).
The aborts overlapped to some degree. Abort selection was based largely on vehicle state at the time of failure and performance. For example, the last possible time to select RTLS ocurred when the vehicle did not have enough propellant remaining to burn its velocity down to zero and then back up high enough to make it to Kennedy Space Center.
This paper gives good detailed information about all I have said.
The reason that the SRBs couldn't be "jettisoned" at any desired time is because the separation system as designed was not powerful enough to safely jettison the boosters if they were delivering a significant amount of thrust. The separation system simply severed the bolts holding the boosters to the External Tank and fired booster-mounted motors to provide clearance between them and the accelerating Orbiter/ET stack. If significant thrust loads had been present, then significant forces and moments would have been applied to the Orbiter/ET stack. It is possible that a more robust system could have been designed, but it wasn't incorporated in the system we had.
Similarly, the ET separation system simply severed the bolts holding the ET to the Orbiter. No separation motors were provided because the system was supposed to be in a low dynamic pressure environment at separation - the Orbiter manoeuvred away using its Reaction Control System jets. In a high dynamic pressure environment with significant propellant sloshing around in the tank, the potential for recontact with the Orbiter was high.
There was some discussion in the other question about the ejection seats used in early Columbia missions. These had limited utility - at a point in ascent the plume expanded out so much that the seat trajectory would pass through it and a "Negative seats" call was made. Similarly, on entry, the vehicle had to be going slow enough that a relatively unshielded ejecting crewmember could slow to a safe velocity.
I deeply wish that a better abort/escape system had been designed in, but that was not the reality of STS.
Edit: I just noticed that the crew training handbooks for intact and contingency aborts are available on the JSC FDF page! These are wonderful references for STS aborts.