What are the functional Difference between the basic three pages which are :- Wiki Page, Publishing Page and a Web Part Page?
Wiki page = the most basic and easiest page of post-2010 SharePoint. You can write text directly in it, have 8-9 different layouts ('boxes' to write and place webparts in), very easy to manage for beginners too. Accepts direct insertion of videos and other multimedia.
Web part page = the oldest type of page (2007 and before) but still present in newer versions. It is composed of fixed placeholders where to put the web parts, does not accept free text in it unless it is inside a content editor webpart. Offers less flexibility than the wiki page but gives you an extra 8-9 templates in case you cannot achieve what you want with the wiki page
Publishing page = activated with the publishing infrastructure, the majority of templates offered by the publishing page gravitates around 'articles'. The most used templates are in the form of a combination of text box, eventual space for fixed size image, byline, date and other common fields that can be later used to pupulate content query web parts/content search web parts. They are projected to be newsrolls pieces (scheduled by date) rather than static pages to contain ordinary intranet material. This type of page can be under publishing control (=approval) and approval with workflow out of the box. Does not offer much flexibility since the space for the image is fixed (left/right).
You forgot the project page and the Enterprise wiki page