It's a new software which tracks your search engine position by crawling your site. Not an ideal way of doing it but many say it's a clever marketing tactic to capture the attention of us webmasters. What better way than to show the name in reports which every webmaster views and takes notice of.
These crawlers are then logged as visits in Google Analytics and show up in your referral traffic report.
It could make a significant impact on your visits and referral traffic reports. Especially for SME's or small site owners.
You could see that referral traffic has increased when it actually hasn't because of these crawlers.
Interesting comments in this post where some are seeing 100 visits per day: http://logorrhoea.net/2014/01/how-to-block-semalt-com-referrer-traffic-using-htaccess/