The only difference, and the most important one at that is, general category is obviously the one without any category reservation, so if their is seats for general category, means only general category students can apply, whereas open category is the one with all the categories combined like general category, SC,ST, OBC etc. so if there is seats for open category, ,means students from any category can apply.

As per your next question, your All India rank in Nimcet is really nice, and you do have good chances of getting in alot of government and private colleges. So my suggestion is, you should definitely wait for the counseling, and participate in that because you have chances of getting the best colleges.

Hope this helps you, Good Luck !!