Color space converter : Convert pixels from RGB to YUV and from YUV to RGB.

fDCT : Forward Discret Cosinus Transform, from spatial domain to frequency domain.

iDCT : Inverse Discret Cosinus Transform, from  frequency domain to spatial domain.

Quantizer : Apply the lossy compression.

ZigZag : Reorder data to achieve a better compression.

Huffman coder : Compact data by representing the more common data with short codes and the less common data with longer codes.

JPEG block diagram -
Each stage is heavily optimized in the Morgan M-JPEG codec