Vejle is the 6th Xbox 360 chip configuration and it sees the CPU and GPU combined, which saved enough space to include built-in WiFi. Hooray.
It's much small and far cheaper to produce as well which helps Microsoft out and us, plus it uses less power. This is always an on-going goal for the platform holders to reduce the power needs and cost of manufacturing the hardware chips.
The former 5 Xbox 360 chip configs were Xenon, Zephyr, Falcon, Opus and Jasper. The new Vejle helps keep overheating in check unlike previous models which could cause the mainboard to warp and thus trigger the dreaded RROD.
If you want to dig deeper into Xbox 360 Slim's Vejle science then check out the lengthy expose over at- they went all super nerd on it.