Mostly trackingcookies, they monitor your internet behaviour, and bombard you with popups of things you have shown an interrest in, your spammail will increase as well.

That's not what Trojans are...

The name come from the Trojan Horse. Trojans are programs that allow someone to remotely access and control your computer. Those programs are usually hidden so you can't see them.

But they could see the Trojan Horse!

Erhm...

They could, but they didn't know what it was. That's why trojans are hidden in other programs.