A .tlk file (or a talk table) is a file containing text to be displayed to the player in Neverwinter Nights. Each piece of text in the file is identified by a number called a StringRef. While text can be stored in other locations, a talk table is useful when the text will be translated, as a translator would need to work with only a single file. A talk table is also useful in reducing bandwidth requirements, as a StringRef is usually a much smaller piece of data than the text it refers to.
The dialog.tlk file is the default .tlk provided by BioWare. This file contains a code telling the game what the player's installed language is (which controls the language of text obtained from a module when there is a choice), and all in-game text provided by BioWare comes from this file (or from dialogf.tlk for some languages when the player character is female).
In addition, custom modules can make use of one additional .tlk file. This custom .tlk file must be placed in the game's tlk directory (not where dialog.tlk is found).