Every Pokémon card has a rarity, usually marked by a symbol near the bottom: a circle for Common, a diamond for Uncommon, and a star for Rare and above. Modern sets go much further, with Double Rares (the "ex" cards), Ultra Rares, Illustration Rares, Special Illustration Rares, and Hyper Rares. The higher tiers are pulled far less often, and that scarcity — combined with popular artwork — is what drives prices.
The gap between tiers is bigger than most beginners expect. A concrete example from real market data: Mega Darkrai ex from the Pitch Black set exists as a Double Rare (#048) at around $0.92, and as a Special Illustration Rare (#116) at around $293.88. Same Pokémon, same set, same playability — a price difference of more than 300x driven purely by rarity and art. Understanding this before you buy anything protects you from overpaying for the cheap version of a card or underestimating the expensive one in a trade.
Eras matter too. The hobby is usually divided into vintage (Base Set through the early 2000s), the middle eras (EX, Diamond & Pearl, Black & White, XY, Sun & Moon), and the modern era (Sword & Shield, Scarlet & Violet, and the current Mega era). Vintage cards like the original Base Set Charizard carry historical weight and are graded and traded as classics; modern cards are easier to find in perfect condition and have spectacular alternate art. Neither is "better" — but they behave very differently as collectibles, and mixing up their logic is a common beginner error.