There are two kinds of backfires. In the old days of carbs, the air cleaner really did two jobs, It cleaned the air and it kept intake backfires from occurring. Gas fumes could come up from the carb and in sufficient concentration that they could ignite. You would hear something between a pop to an serious explosion.
Other other is the more usual exhaust backfire.
If you want a little help, some more information would be helpful. Tell us what make model and year of car as well as any modifications made to it that you are talking about.