Every time you search Google for a business or product and click on a result, the marketers are watching.

Companies bid for spots at the top of the page when consumers search for general terms or even competitors' names. And they closely track where those shoppers end up, analyzing the ways differences in a few words of ad copy or slight adjustments to website layout drive sales.

For Jellyfish, a United Kingdom-based company that has quietly built up a $100 million U.S. headquarters in Baltimore, it is a booming business opportunity. While search engine optimization and marketing isn't an altogether new business, it is an evolving one that is expected to dominate digital advertising spending for the foreseeable future, as more midsize businesses tackle it as a way to proactively connect with customers.

"Companies are getting smarter," said Jay Robertson, vice president of marketing for home alarm system company Protection 1, a Jellyfish client. "There is so much more technological capability today than what we had five years ago."

The company located in Baltimore because work with a large education client here made up a fifth of its business. Within a month, Jellyfish had added GameStop. Now, the list includes Fitbit, Yankee Candle and Skype.

While most of Jellyfish's 250 employees are in the United Kingdom, executives see the United States as their best opportunity for growth because of its massive consumer market and advertising budgets, said Daniel Wilkinson, one of the firm's executive vice presidents.

Spending on digital marketing is expected to reach $67 billion this year and top $100 billion by 2019, growing from about a quarter of all ad spending to more than a third of it, according to Forrester Research. And search marketing is expected to consistently take the largest share of Internet advertising budgets.