Would this be a reasonable summary?
Nominative - Subject of the sentence
Vocative - Invocation
Accusative - Direct object
Genitive - Possessive (of)
Dative - Indirect object (to, for)
Ablative - Where, when, how (by, with, from)
The only other one you are likely to run into (unless you are looking at Latin that was archaic in the time of Cicero) is the locative, but that is used only with certain words or in certain circumstances, and the use is not the consistent
Hence I can say "I am in Rome" and that is "Sum Romae" but "I am in Italy" is "Sum in Italia". Cities, house (domus), the country (rus), small islands, etc get the locative but other places just use regular latin. It makes sense, the locative was dropped but stayed around with common words and phrases.
Of course domus is bizarre itself, since it belongs to two different declensions (it stopped being used so much and then made a come back, and was stuck into the 4th declension...so it has 2nd and 4th declension endings)