Can someone please just give me a basic explanation of what 'grain' actually means?  For instance, what is the main difference between 9mm FEDERAL 115GR. FMJ and 9MM FEDERAL 124GR. FMJ?  I am thinking that  the 124gr. means the bullet is heavier and doesn't have anything to do with the load right?  Is the ultimate goal to balance out velocity with bullet weight? Sorry for these questions, but I am super new to shooting and am seeing a lot of recommendations of ammo types but it's not really meaning much to me.  Thanks.

Grain goes back to more ancient times when things were measured in the weight of grains of wheat.  The more grains the heavier the bullet is.  Not to be confused with grains of powder.

further more the heavier the bullet the slower it is but not always true, bullet manufacturers make some specialty round called +p and +p+ meaning more powder and higher load compression    stick with the baiscs for now
*WARNING* NOT ALL FIREARMS CAN HANDLE +P  AND MOST CANT HANDLE +P+ ROUND MAKE SURE TO ESEARCH FIRST!

try all the regualar facotry ammo and shoot alot of it find what works and go from there  
also FMJ stands for the bullet design, for instance FMJ stand for "FULL METAL JACKET" HP stand for hollow point and there are many many more abbreviations

the best thing to do is go to the book store and buy a good book

Thanks a ton for the info guys.

Grain is a measure of weight: 7000 grains to a pound.  124 gr  refers to a bullet weighing 124gr.

don't feel like you are asking a dumb question just because it might be an easy one.  I have more respect for someone who will ask a "dumb" question than I do for someone who acts like they know what they are talking about but really have no clue

Quoted: don't feel like you are asking a dumb question just because it might be an easy one.  I have more respect for someone who will ask a "dumb" question than I do for someone who acts like they know what they are talking about but really have no clue

Amen.

Quoted: don't feel like you are asking a dumb question just because it might be an easy one.  I have more respect for someone who will ask a "dumb" question than I do for someone who acts like they know what they are talking about but really have no clue

Damn straight! There are no dumb questions... only dumb mistakes!!