Is there a difference between those two words, and what is/are it/they?
Hi! =))
Yeah, there is! 
The rule of thumb is: if the years are from 1 to 4 then ‘god’ is used, if the years are from 5 to 20 then the plural, i.e. ‘let’ is used; the same holds true for the years from 25 to 30, from 35 to 40, and so on! =))
In Russian, 1 or 21, 31, etc. of anything takes the nominative singular, 2-4 as well as 22-24 etc. takes the genitive singular, 5-and more takes the genitive plural, or something like that. Now if that were the only rule you had to learn, Russian would be easy. Note that лет is the genitive plural of год, and года is the genitive singular.
This rule is true of all nouns.
Thanks for all of the explanations! I hear here in Serbia often that Russian is hard because of it’s cases. We have cases too, but Russian are much harder.
I hear that Bulgarian has no cases, the language learner’s ideal Slavic language!
It’s rather a matter of cases in Russian and Serbian being similar but not the same!
I understamd Polish quite well because it’s also a slavic language, but I always make the mistakes as the Pollish has the similar but not the same endings by the declention.
Don’t be in hurry, and you can get accustomed to Russian.