Where pure gold started.
As you know, Mario is the main star of the game. The princess of the Mushroom Kingdom has been captured by Bowser and he's trying to stop Mario getting to her.
This involves you, as Mario, passing through various worlds with enemies trying to stop you at all costs.
You get 3 lives, and if those lives run out, you're dead. Game over. "Oh, so I can just do that level again!" No. "Game Over" in the old NES games really meant game over. You have to start from World 1-1 again if you die. This makes the game hard. Unlike most other games today.
But there are ways such as the Warp Zones, the 99-live trick or just hacking (probably hacking on Wii only.)
On the Wii, you can save. Only by pressing the "home" button. Doing other things and the next time you return, it will start right from that point again.
There is also 2-player mode. If you die, your friend come come in as Luigi and take over.
In the process Nintendo made when taking it to the Wii, they included some acidental glitches and bugs in there as well. You have been warned.
Super Mario Bros. is a good game, it's where gaming gold started. However, it is hard and there are glitches. Also, it is quite outdated. But, worth a go.