In short, Unicode is a character encoding mechanism in which every character is assigned a
unique number, irrespective of the operating system or application. Each language's basic
characters have their own distinct and unique character code values.
What this means with regard to fonts is that it is now possible to have one single font (say
all-in-one.ttf) encompassing characters of very many different world languages.
If your text is communicated in Unicode, your WinXP/2K recipients need not have to install
any Tamil font in their system prior to viewing your Tamil text (For Win98 / ME systems - see here). This paves way for data communication (in Tamil or any other language for that
matter) and storage across computers to be as simple, straightforward and future-safe, as it is
for English.