The Ascent is the period from lift-off to MECO and consists of two stages. During this time frame the Astronauts have nothing to do other then monitor the instruments and hope nothing will go wrong. The Shuttle is on a computer-controlled trajectory to get them to the correct altitude and speed for orbit insertion.
The ascent starts before the shuttle has even left the pad. 6.6 seconds before lift-off the three Space Shuttle Main Engines (SSME) ignite in succession, seperated by 120ms. This allows the computers to verify all three SSMEs are operational and ready to go. Because the SSMEs are off center and the SRBs are still bolted to the pad, the thrust tilts the stack about 2 ft towards the external tank (ET). It is called the “Twang”.
When swinging back into the upright position and the SSMEs produce at least 90% thrust the Solid Rocket Boosters (SRBs) are ignited and the bolts connecting the stack to the pad are exploded and the whole package lifts off. The three SSMEs provide only about 17% of the total thrust at lift-off. The SRBs are doing most of the work. By clearing the tower at around 200ft AGL the Shuttle starts a computer controlled roll maneuver that puts the stack upside down (ET above the Shuttle) and pointing in the right direction for its planned orbit. All the launches are to an easterly trajectory among other reasons to utilize Earth rotational speed.
The first two minutes of flight are intended to move the Shuttle to the thinner atmosphere and to the highest possible speed. This is important due to the fact that two minutes is the nominal burn time for the SRBs. To prevent damage from the vehicle due to dynamic pressure that is growing with speed in the dense part of the atmosphere the thrust of the SSMEs is reduced to around 70% (launch specific) at about 30 seconds into the flight. Throttling up to maximum power occurs at about 60 seconds and maximum pressure (max Q) stresses the Shuttle shortly after it. Because air density is dropping fast with growing altitude the pressure drops in spite of growing speed after that point. 6 seconds after the computers detect that the chamber pressure of both SRBs dropped below 50 PSI the SRBs are jettisoned and parachute down into the ocean where they are picked up by ships. Nominal SRB separation is at about 150,000ft (45km) at a speed of Mach 4.5.
Second stage starts after the SRB separation. During this stage, the Shuttle climbs and accelerates to achieve orbital velocity.
The remaining stack is now much lighter than it was at liftoff due to fuel consumption. The thrust now comes only from the three SSMEs. In the late 90's an OMS assist burn was introduced, after SRB separation the OMS engine light up and burn for about 90 seconds providing some extra thrust.
Some 6.5 minutes after launch the Shuttle performs a second roll-maneuver this time going heads up (since STS-87, Nov. 1997). This maneuver allows communication antennas mounted on top of the Cabin to acquire satellite signals and transmit telemetry back to ground-control.
The shuttle continues on accelerating and climbing until it reaches sub-orbital trajectory. During the last part of the second stage the thrust is again reduced to keep the acceleration at a maximum of 3 g.
Main Engine Cut Off point is where the engines shutdown at about 8:30 minutes into the flight. From there on the SSMEs are just ballast until the next launch. With the SSMEs turned off the external tank is jettisoned, and will burn up in re-entry on it's next orbit. Shuttle's OMS engines will help from here on to reach the desired orbit.