Looking at the definitions given for yeah, yeh, yep, or yup, all those words are defined as exclamation & noun nonstandard spelling of yes, representing informal pronunciation. Looking at the examples provided from the Werriam-Webster Online , it seems that yeah , and yep are used in two different cases. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.