What is the DCT used for in Computer Vision? DCT, what is the benefit of using it for image processing?

I noticed that it is used in JPEG compression.

Can i use it for something else?

Is it useful for motion detection?

Is it useful for compressing further? If I convert an image to its DCT I could remove the higher frequencies to zero, then convert back to image.  Would that make the image size smaller in bytes?

I have ordered books on Amazon about DCT but I cannot wait to start working with it.

In a brief synopsis/paragragh what could I consider DCT of an image for?

Thanks
DCT stands for Discrete Cosine Transform. It is a member of the family of Fourier transforms. Hence it computes/displays the amount and distribution of spectral components of the signal being transformed. Its distinguishing features are that it produces real transform coefficients, it is computed using only real arithmetic and it packs signal energy into low frequency components (thats why it is used in transform based data compression algorithms). For data compression purposes DCT is only an example of other available methods and it serves only the beginnig part of such codecs. You must also consider entropy based coding, perceptual metric evaluation and Quantizer design issues to realize any practical benefits from such transforms.