when you have a bottleneck it means you have a peace of hardware or software that slows down other hardware/software from running at its peak potential. it is term people use to show they have a balanced or optimized PC but its pure BS. there are so many variables in what can cause a bottleneck. your CPU, video card, RAM, mobo, drivers, OS, HDD, PSU, and monitor resolution can all create one. plus if you game, they all have different game engines so a game like Crysis will create different bottlenecks then a game like GTA IV.
so the only thing you should look for, is your PC giving you the results you want. that is the only thing that matters and the only thing you should look for.
Use the Performance Monitor found in Task Manager to assess your PC under different working conditions. If you can put a second screen on for the task then that is very helpful. Otherwise you may have to record the Performance Monitors findings if the application you are testing with demands to run in the foreground. If your CPU is constantly at 100 % or your RAM is hardfaulting a lot or your Disk Queue is constantly high then you are looking at bottlenecks. Use GPUZ to assess the loads placed on your GPU.