M-learning, or "mobile learning", now commonly abbreviated to "mLearning", has different meanings for different communities. The term covers:

learning with portable technologies, where the focus is on the technology (which could be in a fixed location, such as a classroom);
learning across contexts, where the focus is on the mobility of the learner, interacting with portable or fixed technology;
learning in a mobile society, with a focus on how society and its institutions can accommodate and support the learning of an increasingly mobile population.
Although related to e-learning and distance education, it is distinct in its focus on learning across contexts and learning with mobile devices. One definition of mobile learning is: Learning that happens across locations, or that takes advantage of learning opportunities offered by portable technologies.

Growth
Over the past ten years mobile learning has grown from a minor research interest to a set of significant projects in schools, workplaces, museums, cities and rural areas around the world. The mLearning community is still fragmented, with different national perspectives, differences between academia and industry, and between the school, higher education and lifelong learning sectors.

Current areas of growth include:
Location-based and contextual learning
Design of physical spaces (including campuses, conferences, hotel lobbies, cities) to support learning with a mixture of mobile and fixed technologies
Social-networked mobile learning
Mobile educational gaming
"Lowest common denominator" mLearning to cellular phones using two way SMS messaging and voice-based CellCasting (podcasting to phones with interactive assessments)

Scope
The scope of mobile learning includes:

Children and students using handheld computers, PDAs or handheld voting systems in a classroom or lecture room.
Students using mobile devices in the classroom to enhance group collaboration among students and instructors using a Pocket PC.
On the job training for someone who accesses training on a mobile device "just in time" to solve a problem or gain an update.
Learning in museums or galleries with handheld or wearable technologies
Learning outdoors, for example on field trips.
The use of personal technology to support informal or lifelong learning, such as using handheld dictionaries and other devices for language learning.
See m-learning trials conducted in the UK

Challenges
Technical challenges include:

Connectivity
Battery life
Interacting with small devices
Displaying useful content in small-screen devices
Social and educational challenges include:

The intrusion of formal education into daily life: Protecting the privacy of young learners, from being continually monitored and assessed through their mobile devices.
How to assess learning outside the classroom
How to support learning across many contexts
Developing an appropriate theory of learning for the mobile age
Design of technology to support a lifetime of learning