BitLocker is basically its own concept[1], separate from something like Filevault or Truecrypt, and it requires Secure Boot to really do what it does.
BitLocker, although it can be used to encrypt external/data volumes, is primarily a technology for encrypting the boot volume and then storing the encryption key in the TPM chip. That encryption key can itself be encrypted using some combination of a passphrase, a biometric signature, and a smart card, which must be presented at startup.
In the TPM chip, along with the boot-volume encryption key, there is stored a manifest of SHA signatures for important OS files (specifically, the kernel and drivers required to bring up policy-based security like ACLs and domain authentication.) This manifest is signed by that same encryption key. Thus, the bootstrap loader, having retrieved the credentials necessary to make use of the TPM, can then verify the manifest with the key, and then use the manifest to verify that the OS it's going to boot into can be trusted, and will continue to protect the security of the files stored on the drive when control is passed to it.
This whole setup basically means that there's no way to get data off a BitLocker+Secure Boot computer without it allowing it (or, in limited cases, doing tricks with sticking RAM chips in liquid nitrogen.) If you didn't have BitLocker, you could just read the data "at rest"; if you didn't have Secure Boot, you could just install a rootkit and grab the data "in motion."
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[1] Really, I think the only reason the two technologies (BitLocker and Secure Boot) ended up with different names is that they were supposed to be two subfeatures of one initiative -- Microsoft Palladium or http://en.wikipedia.org/wiki/Next-Generation_Secure_Computin... -- but that initiative was shelved (likely due to the huge public backlash), leaving just these few practical remnants behind. (It's pretty obvious that the BitLocker Settings panel was originally the Palladium Settings panel; it's where you go to reset TPM keys et al.)