A state of emergency is a legal apparatus designed to deal with extraordinary circumstances, to restore law and order that has broken down, or to deal with a natural or man-made disaster. It is less extreme than martial law.
Typical of a state of emergency is extra powers for police, or even the use of the army to help "restore order"; and other measures such as curfews or disaster relief. A State of evergency can be declared over part or whole of a country.
South Africa had a national state of emergency from 1985 and 1988 because of the level of "unrest" i.e. violent resistence to the Apartheid regime. During this time the state granted itself great powers of censorship and detention without trial.
A state of emergency as a normal or permanent state of afairs is considered to be characteristic of a repressive police state.