Part I: About authorea 

Difference between Authorea and MS word or other word processors

Writing is the foundation of your academic career \citep{Cameron2009}, and not just academic career. If you work in a knowledge-based occupation, chances are high that you will need to write a variety of documents ranging from memos, letters, notes, to full fledged reports and books. The act of writing depends on tools. Compared to old times when your choices of tools were limited (pen and paper or typewriters), or even as far back as the early 2000s, when your choice of computational tools for writing were limited (plain text editors, and word-processors), since 2010, your options of using the web as an authoring tool has added a new dimension as to the tools you can use to write your documents. Today, you have access to more tools than the basic pen and paper. Unfortunately, the last typewriter was manufactured in 2012 \cite{produced}, although lately, there have  been some revivals.  Importantly, you can use a web page to write your documents and share with your colleagues, friends, and the rest of the world. Authorea belongs to that category of tools. It is a web-based authoring tool designed for writing academic documents where you can blend codes, notes, figures, tables, citations, and you can share your documents with your colleagues, and publish them. You may wonder why we need a web based tool to write documents when we can use tools such as a word processor (MS Word, or OpenOffice or LibreOffice, or Apple Pages), or indeed online word processing apps (for example, Google Docs, or Zoho Docs). The answer is for writing academic documents that have special requirements, a tool such as Authorea that lives on the web is useful in its own way because it is suited to academic style of writing and ported to web, so that we are free to use any computer connected to the world wide web and has a modern web browser.  
Much of our daily writing already happens online and on the web. Every time you post a message on a social networking site such as Twitter, or Facebook, you use an online medium on a webpage or a webapp to write  your text (this could be on your phone or on your computer but you use a browser in some form although it is not always obvious to you that you use a web browser). When you use an app such as Wordpress or Blogger or Medium to post your thoughts in the form of a blog post, you use a web based tool to write it (assuming you use their web-based input). But such texts are limited in scope. Some are limited in the number of characters you can use (in Twitter, you are allowed 280 characters), or that you cannot post a table or you cannot 'embed' a figure in the post. Besides, a tweet, a facebook status update, or even a blog does not qualify as an academic writing (such as a research proposal or journal article or a report).
You can open up a word processing app (example: Microsoft Word) on your computer and start putting your thoughts to the machine, but then again, word processing tools are meant for writing memos, and letters and they do not work as well for long form academic writing. Long form academic writing such as writing a research proposal, or a thesis, or a journal article or requires more than writing free text and adding tables. It also requires that the author should, if needed, embed figures, add equations, graphs, diagrams, and provide citations. Although most of such tasks can be accomplished in modern word processors, you cannot directly share and publish a document to a journal or submit to preprint servers easily and in a time-efficient manner. This is where a tool like Authorea becomes useful. 
To provide you a perspective, take a look at the 'bread crumb' of Authorea (see )