NEWS: Moz changed domain authority algorithm on March 5, 2019. Domain authority 2.0 for first look is better than previous version. Moz spam score and traffic history is included in new DA algorithm.
After checking thousands of domains, in over 90% of cases new domain authority is lower than before. So, even you have very good site, don't be surprised by lower domain authority.
But it's not end of news... I am received information from Moz team, that they have plan to remove MozRank, which currently is completely useless - something like PA divided by 10.
What is your current domain authority after latest change?
Domain Authority is provided by Moz, this metric is at domain level, it meaning that each url/page under one domain will return same value, unlike page authority which is url metric. Both of metrics use logarythmic value from 0 to 100 - more difficult increase DA from 20 to 30, than from 10 to 20.
Page Authority is at url level - it meaning that each url (page/subpage) have own authority and its a part of domain authority. Page authority use also logarythmic value between 1 and 100. Higher value of page authority allow for single page get better ranking in search engines.
In Moz opinion: Domain Authority is Moz's calculated metric for how well a given domain is likely to rank in Google's search results.
In real seo world, domains with high domain authority can have Google penalty or can be banned in Google, so you need also check other signals like Majestic TF, Google index, domain traffic or Semrush metrics - which will tell you how many keywords have provided domain in top search results. Especially when you are looking for expired domains and would like a real increase Google ranking of your website.