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Boot loaders are small programs that manage the boot process. If you've worked with Windows NT, you've had some experience with a boot loader. At startup, NT's boot loader asks what operating system you'd like to boot to.
In Linux, the most commonly used boot-loading tool is LILO, the Linux Loader. During installation (typically at the very end), Linux will generate LILO values and ask you to verify them. At that time, you are given the opportunity to insert additional LILO boot options. For example, perhaps you have additional partitions and operating systems you'd like to add. This way, during system startup you can choose which operating system to use for that session.