Moved from the TCG Competitive Play to the TCG Player Help & Advice as the OP is looking for help on understanding the game.
In the PTCG, cards can be reprinted. This is purely the same card with the same name (for Trainers and Special Energy) or the same effects and same name (for Pokemon) from an older expansion being printed again in a newer expansion. The Parasect you are referring to was first printed in BREAKThrough but got printed again in Generations. Reprint cards are considered the "same card", which means that you can't have 4 Parasect from the BREAKThrough expansion and 4 Parasect from the Generations expansion in the same deck. Reprinted cards also means that you can use copies from expansions that have rotated out of format providing the reprint is in a expansion part of the format. A good example of this would be Double Colorless Energy - someone could use any of the older prints of the card instead of using the recent print.