Are they going to affect my game badly?

They can. It depends on exactly which ones you have, and whether or not they are compatible with your EPs. Most of the time you shouldn't have any trouble with them, though.

Replacement globals and global replacements are two different things. A replacement global is a mod that alters the built-in behavior of something, such as a mod that makes adults leave the swingsets for the kids instead of making a beeline for any nearby swing and refusing to get off until they are ready to pee themselves. They can also be contained in a single item, such as a sign that keeps strays off the lot while it is present. Most of the time you know that it is there because you downloaded it yourself, so you can avoid getting two mods that do exactly the same thing. Sometimes one can sneak into a Sims2pack, though, which is why the Clean Installer is so handy. It will identify everything before you put it into your game. They can cause trouble if they are not set up for your EP set, or if they conflict with each other. Otherwise they can actually be very helpful by squashing some of the behaviors that you don't like or adding behaviors that you would like to see.

A global replacement AFAIK is another name for a default replacement. They replace the EA skins, faces, or whatever with someone's CC. This is usually harmless, unless you weren't able to use the CC in the first place or you have a different set that you wanted to keep. You can only use one of each type at a time.

Thanks! Makes more sense now.