PAL and NTSC.

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As Mirage said, the only thing you'd need to play an NTSC game on a PAL TV is a special cable which will convert the signal.
When you get asked if you want to use 50 or 60Hz, that's the game itself asking you; everything goes black and white with the 60Hz, so you most likely will need the cable. Unless you have another TV which you can play it on.

Still, an emulator is free and you don't need to buy any extra stuff.

When it asks about 50 or 60 Hz, it's between PAL50 and PAL60, the latter of which is different from NTSC. What happens with EU PS1/PS2 is that when you play an NTSC game on it, it actually sends a PAL60 signal to the TV. If you were to play with a US PS2/PS1 on a PAL60-able TV, you'd get garbage on the screen.
In short,
- if you play a NTSC game on a PAL PS1/PS2, you need a proper AV cable for your console, because the default one is crap and lacks the cabling;
- if you play on a NTSC console on a PAL TV, you need a converter to change the signal from NTSC to PAL.