How to Optimize for Google's Featured Snippets to Build More Traffic

What are featured snippets?
Featured snippets are selected search results that are featured on top of Google's organic results below the ads in a box.
Featured snippets aim at answering the user's question right away (hence their other well-known name, "answer boxes"). Being featured means getting additional brand exposure in search results.
Here are two studies confirming the claim:
Ben Goodsell reports that the click-through rate (CTR) on a featured page increased from two percent to eight percent once it's placed in an answer box, with revenue from organic traffic increasing by 677%.
Eric Enge highlights a 20–30% increase in traffic for ConfluentForms.com while they held the featured snippet for the query.
2. Types of featured snippets
There are three major types of featured snippets:
Paragraph (an answer is given in text). It can be a box with text inside or a box with both text and an image inside.
List (an answer is given in a form of a list)
Table (an answer is given in a table)
Here’s an example of paragraph snippet with an image:
According to Getstat, the most popular featured snippet is "paragraph" type:

3. Featured snippets or answer boxes?
Since we're dealing with a pretty new phenomenon, the terminology is pretty loose. Many people (including myself) are inclined to refer to featured snippets as "answer boxes," obviously because there's an answer presented in a box.
While there's nothing wrong with this terminology, it creates a certain confusion because Google often gives a "quick answer" (a definition, an estimate, etc.) on top without linking to the source:
To avoid confusion, let's stick to the "featured snippet" term whenever there's a URL featured in the box, because these present an extra exposure to the linked site (hence they're important for content publishers):

4. Do I have a chance to get featured?
According to research by Ahrefs, 99.58% of featured pages already rank in top 10 of Google. So if you are already ranking high for related search queries, you have very good chances to get featured.
On the other hand, Getstat claims that 70% of snippets came from sites outside of the first organic position. So it's required that the page is ranked in top 10, but it's not required to be #1 to be featured.
Unsurprisingly, the most featured site is Wikipedia.org. If there's Wikipedia featured for your search query, it may be extremely hard to beat that — but it doesn't mean you shouldn't try.
Finally, according to the analysis performed in a study, the following types of search queries get featured results most often:
DIY processes
Health
Financial
Mathematical
Requirements
Status
Transitional
Ahrefs' study expands the list of popular topics with their most frequently words that appear in featured snippets:
The following types of search queries usually don't have answer boxes:
Images and videos
Local
Shopping
To sum up the above studies:
You have chances to get featured for the terms your pages are already ranking in top 10. Thus, a big part of being featured is to improve your overall rankings (especially for long-tail informational queries, which are your lower-hanging fruit)
If your niche is DIY, health or finance, you have the highest probability of getting featured
5. Identify all kinds of opportunities to be featured
Start with good old keyword research
Multiple studies confirm that the majority of featured snippets are triggered by long-tail keywords. In fact, the more words that are typed into a search box, the higher the probability there will be a featured snippet.
It's always a good idea to start with researching your keywords. This case study gives a good step by step keyword research strategy for a blogger, and this one lists major keyword research tools as suggested by experts.
When performing keyword research with featured snippets in mind, note that:
Start with question-type search queries (those containing question words, like “what,” “why,” “how,” etc.) because these are the easiest to identify, but don’t stop there...
Target informational intent, not just questions. While featured snippets aim at answering the user’s question immediately, question-type queries are not the only types that trigger those featured results. According to the aforementioned Ahrefs study, the vast majority of keywords that trigger featured snippets were long-tail queries with no question words in them.
It helps if you use a keyword research tool that shows immediately whether a query triggers featured results. I use Serpstat for my keyword research because it combines keyword research with featured snippet research and lets me see which of my keywords trigger answer boxes:
You can run your competitor in Serpstat and then filter their best-performing queries by the presence of answer boxes:

This is a great overview of your future competition, enabling you to see your competitors' strengths and weaknesses.