The title tag plays an important part in Search Engine Optimisation and indeed Social Media. And indeed for the code that your website is written in to be valid, you need to have a title tag.
Your title tag can’t be too long. No more than 50 – 60 characters and if possibly aim to have it under 55 characters.
This means that 95% of the time the title tag that you write is the one that is shown in the Search Engines.
There are several reasons why Title Tags are useful for SEO.
1. In your Browser the title tag often shows up at the top of the browser itself
2. The title is typically what is shown inside the Search Engine Results page
3. If someone links to your website, which is often the most valuable kind of SEO result, the title tag is often used as the anchor text.
For maximum effectiveness your title tag as mentioned earlier should be around the 55 character mark. Having said that on Social Media sites you often get a longer length, and indeed some search engine specialists believe that even if your title is longer the additional space for keywords is worth having to improve your pages optimization.
What you do need to realize though is that the title tag itself is what will be shown in the results and so even if it is cut off, you should aim to have the most important words shown first.
As a good rule the earlier your main keyword shows up the better it is, and the more likely your end user will see the results.
Also if you have a well known brand, there is often merit to have that at the end of your title.
Although for us the most important rule is that you want to try and get a title that is highly readable and will grab your potential readers’ attention.
References: http://www.w3schools.com/TAGS/tag_title.asp This is the official documentation.