Alpha vs Maxis Match
Electronic Arts has simplified the models used in The Sims over the years, putting more resources into neighborhood terrain detail, Sim models and lots. They have actually reduced the detail in the content we use in lots and on Sims to a fairly low-detail cartoon-like appearance. This is the style we call "Maxis" after the original Sims development team. Any softer, cartoon-like custom content we call Maxis Match as it best fits the style of the base game.
Some people prefer making and/or downloading more realistic content. This tends to be more highly detailed, photographically more realistic content. This content is called Alpha. It's basically just a personal choice with a large fan base for both types! As Alpha carries a lot more detail in some creation types such as hairstyles, we've allowed for slightly higher polycounts.
There is a third bracket often referred to as Maxis Mix - which is a hybrid of the two.