The only difference between a USB and PS/2 mouse is how the device connects to a computer. Mice of both connection standards commonly include a converter so they will work with the other connection type. While USB has superseded PS/2 and is the modern peripheral device connection standard, it's hardly new, and the PS/2 standard is far from dead.
It's All In The Connection
You have two port choices when connecting computer mice to a computer (excluding Bluetooth wireless): USB and PS/2. Computers have varying numbers of USB and PS/2 ports for peripheral device connection; some systems do not include PS/2 ports. Unless you're using a high-end USB mouse with super-accuracy detection, you won't notice a performance difference between the two. Note that wireless USB mice use a USB adapter.
The PS/2 Standard
If you have a PS/2 mouse sitting around from a late 1980s computer, you may be surprised to find out that it'll work with your brand new desktop computer with no legacy adapter. The PS/2 connector first hit the market in 1987 on IBM's desktop PC line of the same name before becoming an industry-wide standard. The 6-pin PS/2 connection type (color coded green for the mouse and purple for the keyboard) is an updated version of the larger 5-pin DIN. Laptops started to drop PS/2 connectors in favor of adding more USB ports in late 2007 and early 2008 following the emergence of the netbook and MacBook Air devices. By the way, any nostalgia you may have for the PlayStation 2 console is unrelated to the PS/2. The forward slash in PS/2 is important because it differentiates the standard from Sony's slash-omitting PS2 abbreviation.
USB Rules The World
USB is the most commonly used computer peripheral device connection standard (sorry, FireWire). The USB standard appeared in 1996 and continues to be a feature on computers, which makes your USB mouse compatible with a lot of devices. USB is a future-proof technology, meaning devices maintain backwards and forwards compatibility with computers that feature both older, slower and updated, faster USB ports. A USB mouse features a rectangular connector and socket that connects only one way. USB connections are notoriously annoying to line up correctly without looking. The USB standard is also used to connect devices like external hard drives, flash drives, microphones, joysticks and cameras.
Adapters Make It Irrelevant
If not included with the mouse, USB-to-PS/2 and PS/2-to-USB adapters are inexpensive and easy to find. Using the corresponding adapter, you can convert one type of mouse connector to the other to make the device compatible with the opposing port type. Attach the adapter to the port on the computer and then the mouse cable to the adapter. You might be wondering why you'd want to use a PS/2 mouse, since USB is newer and better. You can use a PS/2 port for your mouse to free a USB port on your computer for another device, Also, if your USB mouse breaks and you have a spare PS/2 mouse lying around, you just saved yourself a trip to the electronics store.