The game save is something we've all seen countless times. But when I was young, most video games didn't include any sort of save feature at all. If I wanted to beat Mega Man, for example, I had to do it in one sitting. There were a few games, like The Legend of Zelda, that gave players the option to save their progress, but this was generally the exception, not the rule.
Some later games, like Mega Man 3, gave players a password at the end of each level, allowing them to continue the game where they left off. A lot of people would share these passwords with their friends, and sometimes even find web sites that had lists of them (Cheat Code Central, for example). This allowed players to skip right to the later portions of a game. However, passwords proved to be much inferior to save points. A lot of us wrote these things on a napkin or whatever else we had lying about, often losing them easily. Another downside was that our lives and power-up items weren't always preserved. For example, players would get to a level with ten lives, but putting in the password for that level would only start them with three.
The save would eventually become the standard means of preserving player progress, especially in the PSOne era. This was the perfect solution: it allowed software developers to make much longer games without having to force players to complete them in a single sitting, and it gave hardware developers the option to make money selling memory cards.
Of course, these days our consoles come with hard drives, and the size of these hard drives seems to be ever-expanding. We no longer need to purchase memory cards, since we can save our game states right to the console's hard drive.
Hardware isn't the only thing to have changed over the years. The way these saves function has also evolved. The "save point" has been cleverly implemented into many games as part of the actual game world. For example, Resident Evil games use typewriters as save points, as if the characters are writing journals to mark their progress. Red Dead Redemption makes players purchase rooms in which they can sleep to save their games. One of the most interesting ways that save points have been implemented is that of No More Heroes: in order to save, players must go into a bathroom where they "drop a hot steamy save."
In addition to adapting to a save-point-based model, saves have become much more frequent. For example, compare Final Fantasy VII, in which you'll often fight through an hour or two of gameplay between save points, to the more recent Final Fantasy XIII, in which you'll find a save point almost every ten minutes.