As rules and validity of visas might differ based on citizenship, double check with the Indian embassy in your country.
Note that visas are valid from the date of issue, not the date of entry. In addition, beware that you can not switch from one type of visa to another one in India itself (I had to go to Sri Lanka to replace my tourist visa by an employment one. Read about it here)
-Six month validity
-Minimum two month gap period between consecutive tourist visas
-Six month, one year or more
-For visa of more than 1 year and up to 5 years, a letter explaining the nature of business and duration from you company and a letter of invitation from an Indian company might be required
-Up to five years
-A letter of admission from a recognised educational institution with duration of the course might be required
-An employment contract signed by both the parties might be required
-Mandatory when visiting India "primarily to take part in religious activities" (used to combat religious conversion)
Registration with the Foreigners Regional Registration Office
The following groups need to register within 14 days of arrival with the Foreigners Regional Registration Office (alternatively, go to the local police station if the place you are staying at doesn't have one):
-Visitors who intend to stay more than 180 days in India
Update: In 2010, India has introduced a TOurist-Visa-on-Arrival scheme for citizens of Japan, Finland, Luxembourg, New Zealand, Singapore, Laos, Cambodia, Vietnam, Myanmar, Indonesia and Philippines. TOVA Visas are valid for stays of up to 30 days and are available at selected airports (Chennai, Mumbai, Delhi and Kolkata).