Even after many years of being seen as one of the best action role-playing games ever made in video game history, Diablo 2 is still rather compelling in its complexity. Considering the fact this game came out more than a decade and a half ago, that complexity becomes even more astounding, especially when compared with that of newer games. To think that even after all that time, it still doesn't look that bad and outdated compared to more modern ARPGs, most of which were inspired from it.
One of the best yet also most misunderstood aspects of the game is the Runes, which were Diablo 2 items added with the expansion Lord of Destruction. They basically work like gems, but can be socketed with other Runes in a specific order to create Rune Words. These Rune Words have additional properties aside from those of the Runes, which make them very powerful. Runes were so good that when gold soon became hyperinflated, they were used for currency instead. This phenomenon then inspired other action role-playing games to adopt systems that didn't make use of an arbitrary central currency.
However, the drawback is that they do not work on socketed unique, magic, or rare items, which is perhaps called for as being able to turn those into Rune Words can make things way too easy in the game. Just imagine being able to make a socketed unique item even more powerful with a Rune Word; it would be overpowered in the game, if you think about it. You can't even put them in items labeled "Mechanic's," "Artisan's," and "Jeweler's" since those prefixes mean the items have sockets due to that indicated magical property, making them invalid for Rune Words as well.
With that said, Rune Words themselves are very powerful as you can create some incredibly strong combinations of items with them. If you are familiar with Diablo 2, you may be aware that you can use more basic runes to create more powerful runes through transmuting with the Horadric Cube, which is acquired somewhere in Act 2. The recipes are easy enough to find online, but it may take a while for most who are not yet in the know to make sense of them, figuring out which ones are best for their characters at the moment.
An important thing you need to remember with Rune Words is that if the one you want to create consists of 3 Runes, then you must insert it in an item with exactly 3 sockets, no more and no less. If you put it in an item with 4 sockets, for instance, then the Rune Word will not be completed as is. Therefore, you need to be careful with what items you collect to create Rune Words later on as nothing is more disappointing than putting Runes in an item, only to realize you did it wrong. You've then wasted both the item and your Runes.
Also, Rune Words are specific to the items they're supposed to go with. If you're going for a Rune Word for an axe and you put it into a sword, it won't work. Also, don't try it with a socketed Wirt's Leg, even for a joke, since that won't result in a Rune Word either. As long as you remember these pointers, you should be able to incorporate Rune Words for your Diablo 2 character.