Collectors use a few different goals when they chase a set. Knowing the difference helps you decide how far you want to go.
Set complete
One copy of every card numbered in the main set, from 001 to the last number of the main count. This is the usual starting goal.
Set complete with secrets
Every card in the main set, plus the cards numbered above the set size: full arts, special illustrations and gold cards.
Master set
Every printing of every card in the set. That means each card in every finish it was printed in (normal or holo, plus the reverse holo), every secret rare, and for some collectors also the stamped promos and special pattern versions released with the set. A master set can be two or three times the size of the main set.
Living set
One copy of every card that exists for a theme, for example one card of each creature in a series, from any set. Each collector decides the rules.
Keeping track
The hard part of any set goal is knowing what you are still missing. In TCGems, each set page shows how many cards you own and which ones are still missing. Put the missing cards on your wishlist so you know exactly what to look for at your next trade or card show.