The Duolingo English Test is an online exam that you take on your personal computer. Like the TOEFL and IELTS, it tests all four language skills: Reading, Listening, Speaking, and Writing. However, you'll find these skills tested in pretty revolutionary ways!
The test is adaptive, which has some significant consequences for test-takers. Basically, if you answer a question correctly, the next one gets harder. If you answer it wrong, the next one gets easier. This means that, with relatively few questions, the exam can pinpoint your ability level quickly - that's why it only takes around an hour to complete!
Students have been really excited about this new testing option (Take it in your bedroom! In your pajamas! Right now!) but may still hesitate to take it. For example, if you're applying to a ton of colleges, chances are that at least some of them won't accept Duolingo yet, while they're all likely to accept the TOEFL or IELTS.
TOEFL allows you to send your results to four universities for free, while IELTS allows you to send your results to five. After that, there's a fee of $20 per report for the TOEFL and a similar fee (which varies by location) for IELTS. Duolingo doesn't charge for additional score reports. So in many cases, it may actually be cheaper to take the Duolingo test in addition to the TOEFL or IETLS.
Right now, Duolingo is primarily used in U.S. undergrad admissions. However, the test has various purposes. Some businesses have also begun to use it to verify the English levels of applicants, particularly in the hospitality field. Because it's not focused either on academia or the workplace per se, the test has a variety of applications to different fields.
Finally, Duolingo could be a good option for you if you don't do well on traditional English exams. The format is completely different and may be more intuitive for you. Furthermore, the test is a lot shorter - around an hour max, compared to around three hours for TOEFL and IELTS - and that's not even counting travel time for the latter exams!