I am confused every time I heard or encountered a mode operation of computer? What was this really mean?

I know there is a safe mode, a normal mode & etc. Could you help me clear this things out?? Thanks.

Safe mode is is a diagnostic mode. in safe mode you will have reduced functionality, but the task of isolating problems is easier because many non-core components are disabled(turned off). An installation that will only boot into its safe mode typically has a major problem, such as disk corruption or the installation of poorly configured software that prevents the operating system from successfully booting into its normal operating mode.

From Yahoo! Answers

Added -
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Thanks HivelocityDD for your information. It does really help. Thanks again!!