As for Duo, it's just smaller in physical size, and will fit into the smaller Sony cameras ... as the dude said up there ^^ capacity. With Memory Sticks (ordinary ones), you get 8mb, 16mb, 32mb, 64mb ...

Introduced in 1998 with 4MB and 8MB capacities, the original 2" modules looked like a short stick of chewing gum. Sony also designed Memory Sticks as I/O devices for cameras, GPS and Bluetooth ...