what is proc entry?

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http://stackoverflow.com – I am in eager to learn about proc and sysfs entries. I googled to know about proc entries. I can understand that we can create our proc entries to /proc file system. I could not find much about it. I understand that proc entries are the values which is set to proc filesystem. I dont know that which is correctly or not. I want to know that what is the real definition for proc entries and also sysfs entries? where it is used? what is the need to creat it? Please suggest any link to learn about it. (HowTos)