The Game Boy Pocket is a smaller version of the Game Boy. It was released in 1996. The main similarities is that the Game Boy Pocket uses AAA Batteries instead of AA, has a better display, and is much smaller than the original Game Boy. It's also slightly smaller than its successor, the Game Boy Color. It weighs 148g with batteries, compared to 394g for the original Game Boy.
While the Game Boy Pocket is more power efficient than the original Game Boy (3V 0.7W compared to the original's 7V 0.7W), since the Pocket uses only two AAA batteries the battery life is around 10 hours compared to the 15-37 hours that the Game Boy got out of four AA batteries.
The display is 65mm diagonally, slightly smaller than the original Game Boy (2.6 inch, or 66mm), and larger than the Game Boy Color display (2.32 inch, or ~58.9mm). The pixels are true black and white, compared to the Game Boy's green tinted screen. It's also much sharper than the original Game Boy screen and has a faster pixel response time, so blurring and ghosting are less of an issue.
One thing that suffered in the Pocket is the sound output which is very low on the Pocket's internal speaker. It has quieter and poorer sound output than the Color though, especially if using the headphone port. There is a more audible noise compared to the original Game Boy, and when using an AC adapter there's a noticable low pitched hum through headphones.