SG = specific gravity. That is what you are measuring with your hydrometer. Since alcohol weighs less then water and sugar, as it ferments, the SG will drop. That's how you know when your beer is finished- by the drop in gravity to the expected sg reading.
OG= original gravity. That would be the reading you should have had when you finished your brewing, before you added yeast. You have to correct your SG readings for temperature, though, so you have to look at a chart if you take the SG at any temp other than 60 degrees. the hydrometers come with a little piece of paper with that info on it, or you can find it online.
I suspect that your OG was very wrong- you may not have mixed up your wort and top off water enough, and didn't correct for temperature. If you know the correct OG, and the correct FG (final gravity), it is easy to calculate your alcohol by volume (ABV%). Without those readings, it would be a guess. No one can tell you without knowing those readings, or at least the recipe so we can sort of make an educated guess.