http://www.flickr.com/photos/denemiles/3797544572/

Before there was RAM, we had DLM ( Delay Line Memory). There were no semiconductors in the 60s so clever guys had to figure out a different way of storing data. This post in MAKE provides very good explanation and also an analogy for those of us who would not understand the technical lingo:

If you had a hard time remembering things for very long, and happened to live in a cave, you could just shout out what you didn't want to forget, and a few seconds later you would hear an echo to remind you. Of course, the problem with this is that an echo doesn't stick around for long, so you would have to shout again every time that you heard the echo, so that you could remember again in a few seconds. Assuming you could keep this up, you would never forget your idea. Of course, that would get really tiring after a while, so you would be much better off just writing it down.