Power Creep is a phenomenon present in any collectible game that uses both old elements and new ones. The idea behind it is that the company has to sell their new products, but everything new they create has to compete with previously existing pieces. To compensate for this, new cards (or whatever else may be used) end up becoming superior to other cards to the point of becoming strictly superior.
For example, compare Pearled Unicorn with Ronom Unicorn. Both have the same power, toughness, color, and creature type. The newer Ronom Unicorn, however, costs 1 less mana and has an effect, making it superior to Pearled Unicorn in almost every way. The only advantages Pearled Unicorn has are assistance from Muraganda Petroglyphs and cards that look for higher converted mana costs such as the Clash mechanic, both very minor advantages far outweighed by Ronom Unicorn's.
Wizards of the Coast has many policies in place both to maintain balance of the game and to prevent power creep. While some power creep does exist, it is mostly just in comparison to certain cards that never saw competitive play. Gray Ogre and Grizzly Bears are often used to describe cards of the same cost and size with effects. It's become commonplace now for white & green to have 2/2 creatures for 1C (1 generic, 1 colored mana) and the others to have the same size for 2C. The nature of color and mana costs is also very instrumental in preventing power creep, since every color has their own strengths/weaknesses. It's almost formulaic to determine how much any card should cost given the type, creature size, and abilities. For example, cantrips tend to cost 2 more than they usually would as in Kavu Climber vs. Gnarled Mass. Through these measures, each set becomes defined more by the abilities and effects offered that other sets cannot.
Other games aren't so lucky. Dragon Ball Z TCG and Yu Yu Hakusho TCG both suffered from power creep as they went on partly due to the nature of their licenses, but also for the sake of selling cards. Because there was not a constant system of costs, it was very difficult to make new cards balanced, flashy, and powerful without doing the same things older cards had done before.