Shell Programming and Scripting

I have a file with a lot of numbers in it and I need to clean it up and make it look nice and proper. I found this little gem of a one-liner and basically understand what it is doing but I would like to further understand what each part of the command is doing. Being a newb, I am just trying to understand the command that I'm using. I was able to find the command and a brief description of what it is doing but would like to see if someone can break it down for me:

sed -e :a -e 's/\(.*[0-9]\)\([0-9]\{3\}\)/\1,\2/;ta' myFile

Contents of myFile:
4381
1222333
70000
44555666
1234567890
56890
234567
89222
999777
345888777666

Can anyone help me better understand this?

(:a and ta) :a is a label t branch to a

's/\(.*[0-9]\)\([0-9]\{3\}\)/\1,\2/ .... regular expression last 3 numbers and all others,...(1,) print the first RE with , ....2 print the second RE...
THIS PRINT FOR EXAMPLE 1222,333 and the (:a and ta) print 1,222,333

I hope it' clear but i'm french and my english is bad...

So -e :a is the first command that labels it a. -e 's/\(.*[0-9] is part of the second command with the substitution option and it pulls the first set of numbers into the pattern space and counts them. ([0-9]\{3\}\)/ it then counts the number and if it is over three digits it places a comma counting three from the right? Is that about it?