Where do TM's and Term Bases COME from, please?

I'm truly sorry, however I do not understand the language of the help section of SDL Trados at all, so I need to ask:
 
- When I create a translation memory, am I adding examples of my own, previous translations, and these previous documents constitute my translation memory, or alternatively:
 
- Is there some other, outside translation memory to which I can subscribe?
 
- This same question applies to "term bases".
 
- Is a "term base" something that is based off of my own work, or may I subscribe to a service which would permit me to upload an external term base?
 
I ask because I am brand new to CAT tools, and trying very hard to slog through the SDL Studio tutorials and the help sections, but cannot make heads nor tails of the information about translation memory or term bases. Don't get me wrong - I think I understand the concepts, but where do they come from, please?
 
I hope the above makes sense to those who are experienced, and that some patient person can explain it to me.
 
Thanks so much!!
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