Guerrilla gardening is the act of greening a disused or vacant urban space. Because of the laws of property ownership, such an act is usually illegal - and so guerrilla gardening is frequently a clandestine activity, with concerned citizens throwing seed bombs into wasteland or planting up traffic islands in the middle of the night.
There are several reasons why guerrilla gardening is becoming increasingly popular (the term was first used in 1973, according to Wikipedia). The first is that many of us live in an increasingly industrialized and bleak landscape - which is giving rise to many urban problems such as the decay of local communities and an increase in violence. When vacant lots or waste land are used to grow flowers, it is a symbol that people do still care, and although vandalism of guerrilla gardens is common they can greatly improve an area.
Guerrilla gardening to improve the look of a neighborhood is one thing, but many inner city areas have little access to fresh food, and many residents who cannot afford to eat a decent diet. And so some guerrilla gardening projects are more permanent, obvious and organized, and aim to turn unused land in the city into community gardens. The risk with these gardens is always that the land owner will close them down, or that the land will be developed, but the community value of such gardens is increasingly recognized. Not only do they provide healthy food for a disenfranchised community, but they improve the area as well.
The third type of guerrilla gardener is one who wants to make a political statement. Environmentalists want to encourage food production in cities, to cut down on fuel use and pollution problems. But there are also anti-capitalists who want to change the pattern of land ownership and the way the economy is run - and who are attempting to reclaim unused (and privately owned) land for public use.
Guerrilla gardening is non-violent direct action, a proactive protest against how we currently use or misuse land, and about how we can reclaim it for the benefit of the community. The good thing about guerrilla gardening is that is always about making something better - more beautiful or more productive - and as such is frequently tolerated or appreciated where more strident forms of protest are ignored.
Further reading:
http://en.wikipedia.org/wiki/Guerrilla_Garde ning
http://www.guerrillagardening.org/
http://www.pri malseeds.org/guerrilla.htm