A sector is a small part of a platter on a hard drive. It is in these sectors that data is stored. Bad sectors do not store data properly. Bad sectors are often masked by operating systems. So normally it is not easy to detect.
This is especially true if you use a lot of disk capacity. Bad sectors are really bad. You can check the disk yourself. Let me give you an example with Windows 7. The same goes for other Windows versions. Right-click the disk you want to scan and select Properties. Click on the Tools tab. Click Check now. When the box appears, tick both options inside. You can check with the above method.