A BibTeX entry is a series of key-value pairs:

@INPROCEEDINGS{KesslerKlingerKuhn2015,
   author = {Kessler, Wiltrud and Klinger, Roman and Kuhn, Jonas},
   title = {{Towards Opinion Mining from Reviews for the 
      Prediction of Product Rankings}},
   booktitle = "Proceedings of the 6th Workshop on Computational 
      Approaches to Subjectivity, Sentiment and Social Media 
      Analysis (WASSA 2015)",
   year = 2015,
}

As we see, the values can be enclosed in quotation marks (booktitle), single curly braces (author), double curly braces (title) or nothing (year). What is the difference?

• Nothing: can only be used with an integer value afterwards, e.g., a year.
• Quotation marks "...": Special words like ‘and’ in an author/editor field are processed, capitalization is done according to the bibstyle rules (e.g., all caps, all lowercase, …).
• Single curly braces {...}: Roughly equivalent to quotation marks.
• Double curly braces {{...}}: No special words are processed, capitalization is preserved exactly as given.

The exact results will always depend on your bibliography style. Let’s assume we use apalike and natbib. This particular style abbreviates author’s first names, lowercases titles and preserves the exact capitalization of booktitles/conferences. So the above bibtex entry will result in the citation key (Kessler et al., 2015) and the following bibliography listing:

Kessler, W., Klinger, R., and Kuhn, J. (2015). Towards Opinion Mining from Reviews for the Prediction of Product Rankings. In Proceedings of the 6th Workshop on Computational Approaches to Subjectivity, Sentiment and Social Media Analysis (WASSA 2015).

If we were to use single braces or quotation marks around the title instead of double braces, we’d get the citation key (Kessler et al., 2015) and the bibliography entry

Kessler, W., Klinger, R., and Kuhn, J. (2015). Towards opinion mining from reviews for the prediction of product rankings. In Proceedings of the 6th Workshop on Computational Approaches to Subjectivity, Sentiment and Social Media Analysis (WASSA 2015).

If we were to use double braces around the author instead of single braces, we’d get the citation key (Kessler, Wiltrud and Klinger, Roman and Kuhn, Jonas, 2015) and the bibliography entry

Kessler, Wiltrud and Klinger, Roman and Kuhn, Jonas (2015). Towards opinion mining from reviews for the prediction of product rankings. In Proceedings of the 6th Workshop on Computational Approaches to Subjectivity, Sentiment and Social Media Analysis (WASSA 2015).

So clearly you should never ever use double braces around the author. What about titles? Especially when you have abbreviations or non-English titles it may seem a good idea to use double brackets. But be aware that you essentially disable BibTeX’s normalization of capitalization throughout the bibliography. So if at some point in the future you decide/are forced to use a different style that uses different capitalization rules for the titles, you are screwed.

What is the alternative to get correct capitalization in titles without using double braces? Enclose only the letter that should be capitalized in extra braces. For example if we want to capitalize "Opinion Mining" in the title (just because… no real reason) and be sure that "WASSA" is always all caps, we could do this:

@INPROCEEDINGS{KesslerKlingerKuhn2015,
   author = {Kessler, Wiltrud and Klinger, Roman and Kuhn, Jonas},
   title = {Towards {O}pinion {M}ining from Reviews for the 
      Prediction of Product Rankings},
   booktitle = "Proceedings of the 6th Workshop on Computational 
      Approaches to Subjectivity, Sentiment and Social Media 
      Analysis ({WASSA} 2015)",
   year = 2015,
}

And get (Kessler et al., 2015) with:

Kessler, W., Klinger, R., and Kuhn, J. (2015). Towards Opinion Mining from reviews for the prediction of product rankings. In Proceedings of the 6th Workshop on Computational Approaches to Subjectivity, Sentiment and Social Media Analysis (WASSA 2015).

And even for a style that lowercases the booktitle/conference (for whatever strange reason):

Kessler, W., Klinger, R., and Kuhn, J. (2015). Towards Opinion Mining from reviews for the prediction of product rankings. In proceedings of the 6th workshop on computational approaches to subjectivity, sentiment and social media analysis (WASSA 2015).