Selecting Ubuntu in Boot Menu sends me to Grub Menu

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http://askubuntu.com – Sir, I've installed Ubuntu alongside Windows 7. When I selected Ubuntu (for first time) in the boot menu, it opened a GRUB 2 screen. I don't have any idea what GRUB is, and I was expecting that selecting Ubuntu would directly start Ubuntu. I installed Ubuntu in an internal hard disk different from that of Windows 7. (HowTos)