Specialized describes something made, trained, or developed for one particular job, rather than for general use. A specialized tool does one narrow thing very well. A specialized doctor treats one area of the body. Specialized knowledge is deep in one subject rather than broad. Specialized equipment is what you need for a task that ordinary equipment cannot handle.

The useful contrast is with general. Compare 'a general hospital' with 'a specialized clinic', or 'a general knowledge quiz' with 'specialized knowledge of insects'. Specialized points inward and downward — narrower, deeper.

It comes with a quiet trade-off built in, and good writers use this on purpose. Specialized usually means better at that one thing and less useful for everything else. So 'specialized equipment' can also hint at expensive, hard to find, hard to replace. In biology it is used the same way about living things: an animal specialized for eating bamboo is superbly good at bamboo and in real trouble if the bamboo goes.

One spelling note: British writing often prefers specialised, American writing specialized. Both are correct; -ized is safe everywhere and is what most international publishers use. Taiwan textbooks usually follow American spelling.