What does it mean for a paper to be "under review" I dont know much about paper submission as I am not that matured enough.

Can someone kindly say what does it mean for a paper to be "under review"?

First I was informed that it was "Initial version" and now the status says "under review".
How long will the tag "under review" stay?

As I am a Master's student I dont know much about publication.Can someone please say what does the above means and what are the steps that follow in order for a paper to be accepted in a journal.

Do editors send the authors mail keeping them informed about the steps that are being done ?

I will be grateful if someone helps me here.
When you submit a paper to a journal (conferences are a bit different), it first goes to an editor who has some familiarity with the subject area, but may not be an academic. He or she will make an initial determination of suitability and (possibly) quality. If the paper is deemed worthy of examination it next goes to a few reviewers who are experts in the field, usual academics and volunteers. 

Your paper is being reviewed at the moment by this group, who probably don't meet together. They may or may not have your name listed as author. 

When a reviewer completes review he/she writes a report back to the editor. When the editor gets all the reviews back a decision is made (accept, reject, accept with revision, more review,...).

Usually the author is kept informed, but there are often delays in the system as people are busy and not dedicated totally to this one paper to the exclusion of all else. Reviews can take a long time if people are busy with other things. They don't usually have hard deadlines. 

The author will normally get a copy of the reviews, but not the names of the reviewers. The author is usually expected to write a new version taking the comments of the reviewers in to account. Then the paper goes back to the editor who may decide to publish it or start the process over again with more reviews. 

For conferences the process is similar except that the reviewers may be the conference program committee who do meet as a group for acceptance/rejection. Here the process is time constrained by the date scheduled for the conference itself. The committee may also be limited in the number of papers it is allowed to accept to keep the cost of the conference and the resulting proceedings reasonable.