TLD stands for ‘top-level domain’, and it’s used to refer to the last part of a web address after the final dot. Examples include .com, .gov and .org. You can’t have a website without a TLD, and every domain name is made up of a label (i.e. Google) and a TLD (i.e. .com).
The great thing about TLDs is that they usually tell you something about the site that they point to, with the exception of .com which originally meant ‘commercial’ but which is now used by pretty much everyone. Other examples include .org (organisation), .edu (education) and .gov (government).
TLDs can be split into two further categories: country-code top-level domains (ccTLDs) and generic top-level domains (gTLDs).