Hey Serg,
Now, I am not scholar, or I don't have a master and havn't studied years of the Catholic Church. But I will give a visiual that I use to understand, Life after this world.
Okay, Picture this. A young child, is out in the back yard of a house. The child's parents tell him, don't play in the mud, because no mud is allowed inside the house. Well, The child plays in the yard, but did slip a couple times in mud, but still got back up. Though he didn't mean to get muddy, he did feel sorry and geniulay wished to listen to his parents wishes. After a while, his parents called him to the back door. They saw that the child had some mud on him, so they took the hose, and washed him clean. They dried him off and carried him lovingly into the house.
Now, We can both agree that all in heaven is perfect. There is nothing less then perfect in heaven. We are less the perfect. I am not saying that Jesus's sacrafice wasn't enough to cleanse us, but the fact remains, that our souls do still have marks of the sin we commit. Just as sin would be the mud in my little story. Now, Purgatory, is when the child is washed down and cleaned. The parents still love him the same, and they will still allow him to get in, just he needs to be clean. So, If you can picture that, it kinda helps. It helps me, because I am a very visiual learner. I need to see it happen, then I can do it.
I believe it has stayed the same throughout the time, but its a question to put some research into. I hope that little story helped a little.