Wildlife Sanctuaries and National Parks are two of the most common forms of conservation and protection of wildlife. In India, there are 103 National Parks (with a few proposed) and 537 wildlife sanctuaries (source), out of a total of 733 protected areas. While both serve the purpose of protection of wildlife, there are some key differences between the two.
The biggest difference between these two Protected Areas is the restriction in entry. Wildlife Sanctuaries allow certain activities to be conducted within its premises, like foraging, collection of firewood from fringe areas, scientific research and if required, hunting. In National Park, no human activity of any kind can be pursued. Tourism is allowed strictly in the buffer zones and the core zone is completely off-limits except for trained personnel of the National Park.
The boundary of a wildlife sanctuary is not necessarily well defined. A rough area is marked out, and can be changed if required. The change of the boundary requires a 1/2 majority of the legislative assembly of the state. However, the boundary of a National Park is clearly defined and fixed. This boundary cannot be changed, without a 2/3rd majority vote in the legislative assembly of the state within which the National Park is located.
People are allowed to own “immovable” property like land within a wildlife sanctuary. Such persons are given permission to move to and from a wildlife sanctuary. Conversely, no private entity/person can own property within a National Park. Before an area is declared a National Park, the state government must allocate land to people elsewhere, if there is someone owning land within what is to become a national park.
The Chief Wildlife Warden can authorize the killing/hunting of an animal/group of animals under special circumstances within a wildlife sanctuary. This is not allowed in a National Park.
A wildlife sanctuary is generally larger than a national park, and can even contain multiple national parks within its premises.
These differences are, of course, specific to India, based on the Wildlife Protection Act, 1972. They could apply to other countries as well, and there may also be additional differences between these two protected areas, depending on the legislation of the country in question.