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All foreigners entering India need a visa. You can fill it in on line then print the form and the other documents required, and apply by mail. You have to send an application form, passport, 2 photographs, return postage and money order or pay on line, then they usually send you your visa within a couple weeks, and you're all set to go. It is very important that you read the instructions carefully and check you have all the required documents.
Currently (October 2013) in the UK, visas can be obtained through VFS Global. You'll find the information you need, plus application forms at http://in.vfsglobal.co.uk/. The forms have been causing headaches for a lot of people and more information can be found on the various forums.
If you fill them in electronically, you can either then choose to print them and continue as above, or you can make an appointment at any of VFS's offices (in London these are in Hayes, Victoria and Clerkenwell. There are others in Manchester, Edinburgh, Birmingham) to take your passport, photos, and payment. If you choose this option, they will tell you when you're there which day your visa will be ready, and you return to collect it (it's usually a few days), or you can opt to have it returned to you by post, in which case you'll need to buy special delivery envelopes.
The process, if not the addresses for an interview, is the same in the USA. However, there is now also a Tourist Visa on Arrival (TVA).
It may be filled out online, with an uploaded .jpg photo image with exacting requirements, along with quite a few questions that may seem a bit odd: Place of birth of grandparents, for example. So the TVA might seems an attractive alternative, but bear in mind that TVA is much longer to regular line and each visa takes 10-15 minutes to check.
Other countries have different contacts and different requirements for visas.