This is a 9V battery pack with on/off switch and a 2.1mm barrel plug, you get both parts and will have to solder them together. Use this to battery-power your Arduino (or other electronic projects).
This is a really nice power supply. It's a switching DC supply so its small and light and efficient. It is thin so it fits in power strips without blocking other outlets.
It's a nice quad-AA battery holder, with 6" leads.
Sometimes, voltages collide. When you have parts that expect/speak at different voltage levels, you have to build a mediator to smooth things out. This SparkFun BOB-10403 uses PCA9306 two-channel bidirectional voltage-level translator on a nice, small breakout board.