Microfilm is wound on a reel and has to be viewed in a linear fashion where you can progress through the film forward and backward, à la a music cassette tape or VCR tape.

Microfiche places multiple images on a single card in a grid (like 8x8 or 10x10). When viewing the card on a projector you can jump around the grid and move across left to right or up/down or diagonal, allowing more flexibility for scanning. The cards can be stacked which means microfiche takes up less space.