It's English it's called an underdot. It is a diacritical mark much as acute accents and umlauts are. In Unicode it is referred to as "dot below".
In Vietnamese, it's used to represent a particular tone, as Vietnamese is a tonal language where tone affects meaning and so it is good to reflect it in spelling.
Note that in ị the underdot is considered a diacritical mark, but the dot above is not; it is considered part of the i itself.
How to type it depends on your keyboard. One type used in Vietnamese would type it by pressing the key that is in the position 9 is on English-language keyboards, followed by the letter wanted for ạ ẹ ị ọ ụ ỵ (a "dead key" method to diacritics, as on physical typewriters the key wouldn't move the paper). It's combined with other dead keys for ặ ậ ệ ộ ợ ự.
In Unicode, it can be encoded either by "precomposed characters", or by following the vowel with "combinging dot below".
Latin Small Letter I with dot belowand is Unicode U+1ECB: ị. How you type it is system-dependent. Because this is not about English, it's rather off-topic, I think. – Andrew Leach Feb 04 '13 at 16:23