According to the World Tourism Organization, “Sustainable tourism development meets the needs of the present tourists and host regions while protecting and enhancing the opportunity for the future. It is envisaged as leading to the management of all resources in such a way that economic, social and aesthetic needs can be fulfilled, while maintaining cultural integrity, essential ecological processes, biological diversity and life support systems.”

Thus sustainable tourism should:

1) Make optimal use of environmental resources that constitute a key element in tourism development, maintaining essential ecological processes and helping to conserve natural heritage and biodiversity.

2) Demonstrate respect for host communities while contributing to the conservation of their built and living cultural expressions.

3) Ensure viable, long-term economic operations, providing socio-economic benefits to all stakeholders that are fairly distributed, including stable employment and income-earning opportunities and social services to host communities, and contributing to poverty alleviation.

In many ways the above principles of sustainable tourism can be understood as a realization of, and reaction to, failures associated with typical mass-based approaches to tourism.

For more information:
What is community based ecotourism?
What is the history of sustainable ecotourism?