A tower computer is a desktop computer. The strict definition of ‘desktop’ has changed over time, but it is typically understood to mean a computer you use at one spot - i.e. is not easily portable like a laptop.

There’s several sub-types of tower PCs, some of which are actually quite small and not really worthy of being called a ‘tower.’ The ‘micro tower’ is one example, and these can be even smaller than a laptop chassis. Generally though, a tower form factor computer can stand up - which makes it look like a ‘tower.’