Search Engine Marketing or SEM is the process of creating a Paid Search Advertising Campaign for Search Engines (Google, yahoo, Bing, etc.), and since 90% of the searches on internet are done using Google, when you talk about SEM it seems to be just logic to have an immediate relation to Google Ads.
Google offers so many tools to improve your Marketing Plan and SEO (Google Tag Manager, Google Keyword Planner, Google Data Studio, Google Maps, Google My Business, etc.) it makes a lot of sense to focus your Search Engine Marketing efforts on Google.
There are different ways you can set up and measure your Paid Ads, the must common ones are:
CPC (Cost Per Click)
CPM (Cost Per Million Impressions)
CTR (Click Through Rate)
Google will consider two factors when posting your ad out there, your content relevance and your budget.
Content Relevance means having the right content, easy to understand. Google will determine if it provides the right information to the audience you select and therefore it will display it accordingly.
On the other hand, budget also affects your results, the higher your bid the higher the chance your ad will show up. But wait a second, Google algorithm tries to "play fair" for everyone, so no matter your budget, if your content is not relevant the chances for your ad to work will decrease, so really really pay attention to your content.
To explain it better, take a look at this example:
You place a bid of $10 PPC with an upstanding content. Google would give your content a score from 1 to 10, let's say 10 in this case and your budget would get a score of let's say 3.
Your competitor has a bigger budget placing a bid of $100 PPC, but they are not really paying attention to their content, so Google would give them a score of 1 for content and 10 for their budget. So it'd look like this:
You: 3 x 10= 30
Your competitor: 10 x 1= 10
As you can see, your ad would have more relevance to Google even though your budget is not as you wish it could be, but what this means is that, and we insist on it, "Content is Key".