A TEFL Certificate is the basic qualification you need to teach English as a foreign language or teach English as a second language.
The certificate is awarded after you have a taken a short TEFL course. If you take the course online then you can usually take as long (or as little) time as you need to complete it. If you take the course in-house it is usually about 4 weeks long.
Certificates are awarded by different schools. For example, ICAL TEFL offer the ICAL TEFL Certificate Course after trainees successfully complete an online course; Cambridge offer a TEFL certificate (called CELTA) after trainees successfully complete a 4 week in-house course.
There is no regulated curriculum but most courses will include modules on lesson planning, classroom management, language teaching theory and so on.
Currently there is no single body which regulates the content of courses and the standard to which trainees must meet in order to be awarded a certificate.