What is Abandonware?
Abandonware is software that has been abandoned, that doesn't receive more any more updates or support, and as such may gave you trouble when you try to run it on modern operating systems and hardware.
This term may refer to a project that has been abandoned by its developer despite its popularity, or to software that is simply old and the developer has moved on to other projects.
One interesting example is that old games had their speed based on the speed of the CPU. When the CPU's were slow, the games ran on their intended speed. Modern day CPU's are much faster than old CPU's, so some games run so fast they have become unplayable, specially as there were no updates to fix this behavior. To play games like this, or to run similar abandonware, workarounds are necessary.