Wikipedia can help too :) https://en.wikipedia.org/wiki/Finite_verb
Anyway, if I take English as an example, the finite verb is the verb that is usually close to /next to the subject, and indicates the number of persons/objects of the subject, and the time.
I am employed by a university => "am" is the finite verb, as it gives information on the subject (meaning that a single person, namely I, is the subject), and that the sentence says something about the present. Contrast
They were employed by a university => since the finite verb changed to 'were' , we know that the subject must either be a 'you' or a plural (he/she was, I was...), and also that is is about the past. "employed" does not change.
The main verb is the verb that gives information on what kind of action. In the above cases "employed" would be the main verb. After all, "am" or "were" does not give any information about what I am doing. If I'd say "I am surfing", you'd probably get my meaning if I'd say "I surfing", but you would not get useful information if I left out 'surfing' and just said 'I am'.
In many cases, the finite verb is the same as the main verb, for example in sentences that only have one verb at all: "I like apples", "I love you", "You walk fast!"
That being said, I believe that main verbs or finite verbs are mainly useful in grammar tests; if you read enough in a language and talk about it, you'll see the logic of the language, even if you may not be able to give the exact grammatical terms :)