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The ISA bus is the standard (slow) I/O bus still used in virtually all PCs today for low-bandwidth peripherals such as modems. Older PCs, typically those from before 1993, use ISA-based video cards as well.
ISA video cards are quite obsolete. Due to the limitations of the ISA bus, they are extremely slow by today's standards, and not suited to the use of a graphical operating system like Windows. They also, due to their age, tend to be more limited in terms of acceleration functions, and have less memory on board as well. While it is possible that a high-end ISA card can be a decent performer if it has an advanced set of on-board acceleration features, in practice even a cheap PCI video card will generally outperform even a good ISA card, and by a lot. They are adequate for many DOS uses, character-based applications, and some games, but even there you're really stretching it.
Unfortunately, most PCs that use ISA video cards use them because the ISA bus is all the motherboard supports. In this case, upgrading to a VESA Local Bus or PCI video card requires a motherboard upgrade, which is a significant job. However, some cheap 486 PCs, especially from the 1992-1994 era, support VLB and yet shipped with ISA video cards (to save money). With VLB now itself basically obsolete, an upgrade to a VLB video card in such a system can yield very substantial improvements in performance, at a very low cost!
ISA video cards still have one important use even today--they are helpful with debugging. Since today's video cards are so complex, they can sometimes cause conflicts with other hardware, or in other ways be "part of the problem" when the system won't boot. In this case, temporarily substituting a "plain vanilla" ISA video card may help you determine if it is the video card or another part of the system that is causing difficulties.
Next: VESA Local Bus