The phrases “digital marketing”, “online advertising” and “internet marketing” do seem similar but there are some differences.
Digital marketing is much broader and includes any type of messaging that can be delivered digitally such as text, image, audio or video on websites, in apps, in emails and in ads.
Whereas “online advertising” is paid advertising to providers so you can reach audiences outside of the platforms you already control.
For example, email is a form of digital marketing, because its a method of communication with your customers, but if you were to purchase an email list it becomes online advertising because you are paying for access to that new audience.
We commonly think of our access to the internet through our desktops, laptops, tablets and smart phones, but these days this can include media that has recently become digital such as digital billboards.
More devices are becoming “smart” and joining the internet, so the number of devices (and surfaces) that are online is increasing exponentially. The next fridge you buy may be smart, so you can expect to see ads on your fridge door (this fridge might even be discounted to compensate you for the ads you’ll see).