The ad auction process takes place every single time someone enters a search query into Google. To be entered into the ad auction, advertisers identify keywords they want to bid on, and state how much they are willing to spend (per click) to have their ads appear alongside results relating to those keywords. If Google determines that the keywords you have bid on are contained within a user’s search query, your ads are entered into the ad auction.
Digital marketing is defined by the use of numerous digital tactics and channels to connect with customers where they spend much of their time: online. From the website itself to a business's online branding assets -- digital advertising, email marketing, online brochures, and beyond -- there's a spectrum of tactics that fall under the umbrella of "digital marketing."