>>730
Subject or object; that in the sentence which is doing or being something. You can think of 'ga' as a very firm, very necessary, very connective 'wa' that establishes whatever it marks as vital to the construction of the sentence.
Another helpful tip: The 'wa' particle can almost always be eliminated from a sentence without its "meaning" changing in Japanese, and it often makes for me "natural" Japanese. However, a sentence that has a subject in Japanese (as indicated by the 'ga' particle) MUST always have a subject to achieve the same meaning.
Your example:
ボブは魚が好きだ - with topic
(ボブ・私・大統領）「は」魚が好きだ - without topic
Same meaning basic meaning.