In the image above, imagine this is a plot of land delegated to you. You’ve decided to build a few houses on that land that each serve different purposes. There is your main house, or root domain (“example.com”), a house specifically labelled for a blog, a space related to teaching, and a space related to history (whatever those may be).
These can all connect to each other or not, based on whether or not you post the specific link anywhere.
If you were to want to take your site elsewhere, you can pack up everything (house and/or address, content and/or domain) and take it to a new web host. You can even choose select sites to take with you and others to leave behind.