Questions

Email header is an information attached to each email, which contains the details of who sent it and what route was followed. It can tell you if an email has been forged.
How do you view the headers depends on the email program you are using. In Outlook Express, for example, select the message by clicking on it in a list (without opening it), then go to File>Properties>Details
In RoundCube for headers press the small down-pointing arrow on the right.

Here is an example of a header, you can see that the message pretends to be from Government but really it is not:
Return-path:
Envelope-to: info@netcells.net
Delivery-date: Mon, 13 Jan 2014 10:00:01 +0000
Received: from [36.83.109.134] (port=51008 helo=aexp.com)
by server.netcells.net with esmtp (Exim 4.82)
(envelope-from )
for info@netcells.net; Mon, 13 Jan 2014 10:00:01 +0000
Received: from 36.83.109.134(helo=uaqrfmp.ymnaxbekgtlpu.biz)
by with esmtpa (Exim 4.69)
(envelope-from )id 1MMSYV-3581uy-X9
for info@netcells.net; Mon, 13 Jan 2014 17:00:01 +0700 Received: from 36.83.109.134 (account fraud@aexp.com HELO inupholh.houvqtyebqcp.info)
by (CommuniGate Pro SMTP 5.2.3)
with ESMTPA id 789071713 for info@netcells.net; Mon, 13 Jan 2014 17:00:01 +0700
Date:Mon, 13 Jan 2014 17:00:01 +0700
From:"gateway.confirmation@gateway.gov.uk"
X-Mailer: The Bat! (v3.0.1.33) Educational
X-Priority: 1 (High)
Message-ID: <0585609717.G2RLW408290337@ylzucv.dgaqsuvb.tv>
To:
Subject: Your Online Submission for Reference 435/GB7823986 Could not process
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="----------32693788BDFE836"