This is a simple 4 cell AA battery holder. The 5 inch cable is terminated with a standard 5.5x2.1mm, center positive barrel jack connector.
The connector mates with the barrel jack on the Arduino (among a number of other products) allowing you to easily make your project battery powered.
Note: the average voltage regulator has about 1V of dropout (but can vary greatly). This pack, with normal alkaline batteries, will output ~5.5V causing a normal 5V board to run at around 4 to 4.5V. This depends a lot of what board and processor you are using with the battery pack. Please consult yer datasheet.
Miniature Single Pull Single Throw switches. These are high quality Omron type B3F momentary on switches. This switch has a very large 5mm tall button. Perfect as an embedded switch behind an enclosure wall. Rated up to 50mA.
LEDs - those blinky things. A must have for power indication, pin status, opto-electronic sensors, and fun blinky displays.This is a very basic 5mm LED with a red lens. It has a typical forward voltage of 2.0V and a rated forward current of 20mA.
This is the P2N2222A, an NPN silicon BJT (Bipolar Junction Transistor). This little transistor can help in your project by being used to help drive large loads or amplifying or switching applications. The P2N2222A is specifically rated at 40V and 200mA max