Machine spirits, there's a duality there. At one side, the concept is basic into the Mechanicum belief systems. Machine spirits are within everything mechanical and artificial. Your car have a machine spirit, summed up from the machine spirits of it's countless individual pieces. A lasgun have one too. As do starships or something as simple as an user interface terminal. I think some would consider the OS of computers the respective machine spirits.
Machine spirits reside within any construct (be the spirit interactive like the case of a computer or not like say a mechanical pencil) and are to be pleased and respected if you want equipment to keep working properly. Not only maintenance is needed, prayer and healthy respect are a must too.
On the other hand, when the machine spirit actually manifest back beyond a simple computing program, then machine spirits are AI, be it brains with chips or chips with brains, the result is the same, no matter if it's stored in hardware or wetware, they are very advanced programs, or artificial intelligences. However they are what you might call dub AI, as in non-sentient AI. It's programming far more basic from one of those true complete AI.
These machine spirits tend to follow seemingly basic algorithms but they are capable of perform very complex actions to follow said algorithms, like the case of the Land Rider, which function could be summed up to "protect the imperium", "kill the xenos" or whatever you might come up. There's a instance of one such vehicles waging guerrilla warfare against Orks no less for a few weeks if I recall correctly.