What is GRUB, GRUB2, and how to restore it after Windows installation (Linux)

When you power on your computer, the first software that runs is a bootloader that invokes the computer's operating systems.

GRUB, the GRand Unified Bootloader, is an integral part of many Linux systems. Is a program that installs a boot loader to the MBR

GRUB 2 is the next generation of GNU GRUB. GRUB 2 is a complete rewrite and is at a developmental phase. The most important goal is to make GNU GRUB cleaner, safer, more robust, more portable and more powerful.

Ubuntu 9.10 (Karmic Koala) started using grub2.

Windows installation destroys the grub, so you don't have the operating system menu choice on boot if you install Windows after a Linux OS.

This is a nice guide how to recover grub2 after Windows installation

In addition if you wanna change the default OS boot, the timeout in bootloader menu or bootloader appearance (and more), an easy way to go (at least for ubuntu users) is StartUp-Manager.
StartUp-Manager configures some settings for grub and splash screens (Currently only Usplash). It provides an easy way to use interface.
You can download via Synaptic Manager, Ubuntu Software Center or from Terminal using the following command:

sudo apt-get install startupmanager

Read about GRUB2 Basics (ubuntuforums.org)
More about GRUB (gnu.org)

links
Linux.com: Getting started with GRUB