Trojans are files that pretend to be different from what they are; they look nice, but are in fact malicious. A very important difference from real viruses is that they do not multiply themselves, as viruses do. Trojans contain malicious code that, if activated, can lead to loss or even theft of data. You actually have to invite trojans onto your computer, for example by opening an e-mail attachment, to spread trojans. PWSteal.Trojan is a Trojan.