Discs are flat plate like thing that when they fly through the air they are more like saw blades. depending on the gauge determine how thick they are and how many will be in the shell. Getting hit by one would be nasty. Pellets are small round balls. In shotshells the smaller the number the bigger the pellet. Just like in ratting the Gauge size till you get to 410. 410 is classed by it's bore size that is why you see it call .410 bore a lot of times because that is the diameter of the bore. where the others were classed by how many balls of a given diameter. I my self shoot a lot with the 10 Gauge I could go on and on all day about shotguns. But I hope this is what you were looking for if not let me know and I will try explain it better. please give me a positive feed back.
You would want to use more pellets. I don't think that you would have any luck getting pheasants with what the .410 would have in it most people use 12 gauge with 1 1/4 of shot in size 4, 5, or 6. Some people myself included late in the season when the birds are getting more wary will fly sooner I like Remington's 1 1/2 load of 4's the shells are a little more money but I can get my limit.
I forgot to tell you yes the pellets are the small lead balls.