The ? Block is a classic item that has appeared in almost every Mario game made. Inside "?" blocks there is always a surprise such as a star, a coin, or a mushroom. The easiest way to obtain the item inside is to strike them from underneath. The general purpose of a ? block is to give Mario or Luigi an item of some sort that will help them. They debuted in the game Super Mario Bros. and have taken on many various appearances over time. If you hit them while Mario is in Super form or when he got affected by Super Mushroom, you'll always get Fire Flower. Some blocks have wings. Blocks with wings are always red.
The ? Block first appeared in the NES video game Super Mario Bros., where they would appear in almost every stage. Once you struck it by jumping, either a Mushroom, a coin, a starman or a Fire Flower would be released. Oftentimes you'd find a ? block that you could hit multiple times, thus giving you a heft amount of coins. According to creator Shigeru Miyamoto, this was at first a glitch that occurred in the game, though they liked the idea so much that they basically implemented it throughout the many levels.
If an enemy happened to be on top of a ? Block, then you would be able to jump underneath it to kill it, similar to the ability found in Mario Bros.
The ? Block didn't return in 'Super Mario Bros. 2 (American version), though this is probably because it is a remake of a non-Mario video game called Doki Doki Panic. However, in Super Mario Bros. 3, you'd once again be able to find all of the items that you once were able to, among others such as the Raccoon Suit.
In Super Mario World, you would be able to find Yoshi Eggs in the ? Blocks, which would eventually hatch into a Yoshi. In the 3-D titles, they weren't as populous as they were in the 2-D series, though this isn't exactly the case with Super Mario Galaxy on the Wii, where you could find star bits and coins.
In New Super Mario Bros., the ? block returns. You can find all of the previous items mentioned, with the exception of Yoshi Eggs, Raccoon Suits, star bits and some others. Instead of those, you could find a Mega Mushroom or a Mini Mushroom inside it. But they don't contain multiple coins anymore. The classic brick contains it, and if you collect all of the coins in time, a Super Mushroom will pop out.
The ? Block is basically a yellow box with a large white question mark on the front of it. The appearance hasn't changed much over time, something that can't be said about most of the other Mario series objects.