Informal learning is the learning that is unstructured, unorganized, spontaneous, and in some ways accidental. There may not be any specific objective for this kind of learning but at the end the learner gains knowledge about whatever s/he was skimming through. Informal learning can happen anywhere at work, while interacting with peers, or being a part of real time discussion.
As opposed to formal learning which is structured and follows a particular curriculum and which has a specific objective; informal learning does not follow any curriculum. Informal learning need not necessarily be work related. It can be anything, attitude, any kind of language, day-to-day experience, skills that mat or may not cater to your immediate need.
But as it is claimed that learner movitivation is central to any kind of learning, I believe learners who are highly motivated take up a lot of informal learning on their own. Because a lot of learning happens in an informal situation.
Jay Cross describes "Informal learning is like riding a bicycle: the rider chooses the destination and the route. The cyclist can take a detour at a moment’s notice to admire the scenery or help a fellow rider." Where as "formal learning is like riding a bus: the driver decides where the bus is going; the passengers are along for the ride. People new to the territory often ride the bus before hopping on the bike."
In an informal learning environment the learner feels that s/he has more control of what they want. The content is not pushed or thrust on them as opposed to formal learning sessions. It is also cheap and inexpensive as compared to the amount of money that is spent on formal learning.
Hence organizations should encourage learning that happens at the workplace, without compelling them to take up training sessions. Because a lot of learning happens in an informal environment.