Ad tech firm TubeMogul, co-founded by Brett Wilson and John Hughes, provides an all-in-one platform for clients who wish to create video ads and then track the success of those advertising campaigns across desktop and mobile devices. As more consumers watch videos on devices other than televisions, TubeMogul believes that its ability to provide real-time analytics will set it apart from other video ad tech competitors. However, for most of the company's history, the majority of TubeMogul's revenue has come from advertisers who are creating ads for desktops; the company could soon find itself needing to catch up with competitors who already focus on mobile. TubeMogul hopes to increase its revenue over the next few years by adding digital advertising capabilities for other formats, such as social media. Note: employee count as of Dec. 31, 2015

Founded by G. Cotter Cunningham, RetailMeNot bills itself as a "digital offer marketplace." On RetailMeNot's national and international websites, consumers can find coupons and savings deals that are redeemable at a variety of restaurants and retail stores. The company says that by building partnerships with more than 12,000 retailers and working with them to create offers that are exclusive to RetailMeNot's site, it's been able to set itself apart from other digital coupon sites. But in 2015, traffic to RetailMeNot's desktop site began to fall from the previous year, which could indicate trouble ahead. RetailMeNot says it plans to invest more money into developing personalization tools and other features to drive more customers to its website. Note: employee count as of Dec. 31, 2015

In 2004, Tobias Lütke, Daniel Weinand, and Scott Lake, owners of an Ottawa-based ski and snowboard shop, wanted a more personalized way to sell their products online. So they created Shopify, a software company that provides customers with a bundle of tools to help create their own e-commerce store. The low cost meant that small businesses--not just gigantic retailers--could now easily have an online presence. But Shopify is now facing competition from tech giants like Amazon and Google, as well as newer startups like Square, whose payment-processing platform rivals one of the many services Shopify offers. Shopify continues to add more features to its platform in a bid to stand out. Note: employee count includes total number of both employees and contractors, as of Dec. 31, 2015

Mikkel Svane, Alexander Aghassipour, and Morten Primdahl founded software provider Zendesk to help customer service keep up with the cloud. Zendesk's software assists companies in managing customer-service requests that come in from a variety of channels, including email, the Web, social media, and the old-fashioned telephone. The company also has a variety of apps that companies can embed in their websites. However, the customer-service software industry has relatively low barriers to entry, so Zendesk has many competitors, including Atlassian, which raised nearly $500 million in preparation for its IPO at the end of 2015. But Zendesk believes that its competitive advantage is software that makes intercompany communication easier. Note: employee count as of Dec. 31, 2015