Short answer: eight giant bolts.
Long answer: the Space Shuttle is attached to the External Tank (ET) which is attached to the two white Solid Rocket Boosters (SRB) which is bolted to the mobile launcher platform (MLP) with eight 28″ long (3.5″ diameter) bolts, four on each SRB. At T-6.6 seconds the Shuttle main engines (SSME) ignite, and then at T-0, the SRBs ignite; the bolts are detonated releasing the combined Shuttle stack.
If you look at the below photo of the space shuttle Enterprise sitting on the MLP, you can see how the orbiter hangs from the external tank which is attached to the solid rocket boosters:
Below are photos of the SRBs and the hold-down posts (all photos by Steve Patlan):
Also note that when the main engines ignite, the direction of thrust is off vertical axis, which causes the shuttle stack to do what is called the “Shuttle Twang,” tilting forward and then settling back vertical (this is why the main engines fire at T-8 seconds). Here’s a video of the external tank in real-time:
Update: Here’s a photo from the bottom of the flame trench of the business-end of the SRB. Note the two Sound Suppression Water System (SSWS) nozzles on the left side.
This is another in a series of posts documenting my time around the liftoff of STS-134 Space Shuttle Endeavour. For more posts, check out my STS-134 Launch Page.