X.25 is packet based switching and indeed is very similar to Frame Relay which was a development of it. The principle difference is that it was used at a time when links were more prone to errors and therefore has far greater error correction abilites both at the link and network layers. The downside to this is of course the considerable overhead that is added to the data.
Its still used in places but they are mostly legacy or niche applications such as X.25 over ISDN D channels. Security is no different to Frame Relay.
