transliteration vs. romanized keyboard

I am still trying out numerous Tamil applications online. Now I am
not sure if some of these would be called a transliteration system
or a romanized keyboard. Many have two to one mapping and what is
called "non-transparent transliteration". This is labeled a
tranliteration prgram but roman letters are not seen on the screen.
Some are called romanized keyboards but do the same thing.

Is there any definition or scientific way to differentiate bewteen
transliteration and a romanized keyboard or is it a continuum?
Basically, are they two different phenomena or varieties of the same
phenomenon?

Suzanne McCarthy