Mirroring monitors means that you see the same thing on multiple monitors.
Spanning monitors means that your computer interprets all of the monitors that are spanning as one giant monitor. You can see different things on each monitor, and your desktop, taskbar, Start menu, and background are stretched across the monitors.
Extending monitors means that your computer interprets the monitors that you are extending as separate devices. You can see different things on each monitor, and your desktop background is displayed on each monitor.
In general, video splitters can only be used to mirror monitors. If you want to extend your desktop across monitors, you need to make a separate video connection for each monitor that you are attaching. If you want to attach more monitors than your computer has video ports for, you can read about USB Video Adapters here: www.startech.com/AV/USB-Video-Adapters/.
If you are using a video card with a DisplayPort connection that supports DisplayPort 1.2 you can use MST hubs from StarTech.com to extend your desktop to each monitor.