Lightroom reads the metadata for each one of your images, which includes your camera's make and model, the date and time the image was taken, and the settings you used when capturing the image ( aperture, shutter speed, ISO etc.) All that information is stored in a "catalog," which is Lightroom's word for its database. This makes it easy for you to search for and find your images without having to manually page through all of the directories your hard drive. Lightroom is database-driven, which means that it catalogs all of your images in an intelligent way, using keywords and metadata to sort and store information about each photo. Its primary function is as a photo organizing tool. The reason you can't do any of those things is because Adobe Lightroom's secondary function is as an image editor, and a basic one at that.
You can't merge photos shot at different exposures to create a high dynamic range image, you can't cut one object out of an image and paste it into another and you can't use any of those cool artistic filters that Photoshop users have come to love (or hate). You can't use it to make yourself lose 30 lbs, for example, which is something you can do (given enough time and skill) in Photoshop. the much more expensive Photoshop, currently priced at $699), Adobe Lightroom can't do most of what Photoshop can. They do have a few features in common, but they are actually good for vastly different purposes.
Part of the confusion comes from Lightroom's proper name: Adobe Photoshop Lightroom, which should in no way imply that the two pieces of software perform similar functions. Now if you're not already familiar with Adobe Lightroom, I bet you thought it was just a scaled-down image editor, similar to but nowhere near the equal of its big brother Adobe Photoshop.