A lithium-ion battery pack offers a decent length of play time (playing graphics-intensive games or movies will drain the battery faster than playing music with the screen dark) -- just don't expect it to last as long as your Gameboy without recharging.
The AC adaptor, of course, allows you to play and charge the battery at the same time.
PSP Hardware Specifications
Here's all the technical info about just what the PSP has inside and out.
Color: black Dimensions: 6.7"/170 mm wide x 2.9"/74 mm high x .9"/23 mm deep Weight: .62 lbs/280 g (including battery) CPU: PSP CPU (1-333 MHz) Memory: 32 MB Main Memory 4 MB embedded DRAM Display: 4.3", 16:9 widescreen TFT LCD screen 480 x 272 pixels 16.77 million colors maximum luminance 180/130/80 cd/m2 (when using battery pack) maximum luminance 200/180/130/ 80 cd/m2 (when using AC adaptor) Sound: built-in stereo speakers headset connector Connectors, Ports and Drives: IEEE 802.11b (Wi-Fi) USB 2.0 (mini-B) Memory Stick Duo infrared DC in 5 V connector headset connector UMD drive (Read only) Buttons and Switches: d-pad analog nub triangle, circle, cross, square right and left shoulder Start, Select, and Home Power/Hold display brightness, sound, volume up, volume down wireless LAN on/off UMD latch Power: lithium-ion battery AC adaptor Access control: UMD region coding Parental control UMD (Universal Media Disc) Specifications
Dimensions: 65 mm tall x 64 mm wide x 4.2 mm deep 60 mm disc diameter Weight: 10 g Capacity: 1.8 GB (single-sided, dual layer) Wavelength: 660 nm (red laser) Encryption: AES 128-bit