VT-x is Intel's name for its virtualization technology. Simply put (and I can't put it any other way), a processor with VT-x will be better at virtualization - i.e. running virtual machines using software like Oracle's VirtualBox or VMware's VMware Player.
If you have an Intel processor, you can easily find out if it supports VT-x. First, find out the exact processor type and model number. In Windows this can be accomplished by right clicking on "My Computer" (Tento počítač, Počítač) and choosing "Properties" (Vlastnosti). This will open a window showing "vital" information about your computer. On my computer, under System there is a line that says:
Procesor: Intel(R) Core(TM)2 Duo CPU T5670 @ 1.80GHz 1.80 GHz
To get the same information in Linux, you could do:
# dmesg | less
Armed with this information, I would say to go first to Intel's site and use it's search box, but - sadly - this is not the easiest path to the goal. In fact, it's a good way to waste a lot of time and not find anything useful. Instead, type the following into Google:
core2duo t5670 specs
In all likelihood, the first page listed in the search results will be current the Intel page with specifications on the Core 2 Duo T5670 processor.
Near the bottom of the specifications table, under "Advanced Technologies", it says (in my case):
Intel® Virtualization Technology (VT-x) No
So I'm out of luck. But I can still run VMs - they just run slower and I can only present a single core to the guest OS.