Entrance examinations to Medical colleges are conducted on an All India basis as well as state wise basis. Apart from this, some national level institutions also conduct their own entrance tests. The test is based on 12th standard syllabus.
Eligibility : 10+2 / equivalent with Physics, Chemistry & Biology, or Pre Medical or B.Sc. background.

The Central Board of Secondary Examination, Delhi conducts an All India Pre-Medical/Pre-Dental Entrance Exam to select the candidates at an all India level (except in the states of Jammu & Kashmir and Andhra Pradesh ) for admission to roughly 15% of the total  seats in the colleges of each state. Andhra Pradesh and Jammu & Kashmir do not admit students from other states. Information about the exam can be obtained from the following address: 

All India Pre-Medical/ Pre- Dental Entrance Examination Unit
CBSE, 17-B, Indraprastha Estate, New Delhi 110002

The states also conduct their own Pre-med entrance exam for domiciled candidates. Gujarat and Maharashtra admit students on the basis of merit, in the 10+2 examination.
There are some national level institutes where admission does not hinge on domicile status. These institutions admit candidates through their own Entrance test.

Time of Entrance tests:
The announcement for All India Pre-medical Examination comes  in January and tests are conducted in March or April. 
State Level examinations are announced in January and conducted in March-April. 
National level institute examinations are announced usually in January- March and test conducted  in  May-June. 

Merit ranking of this entrance exam is applicable for admission to MBBS, BDS (Dentistry), BHMS (Homeopathy), BAMS (Ayurveda). 

Syllabus Of Some of the entrance tests