What is the difference between a social share and an ad on Twitter?
A social share is the text and graphic that appears on Twitter when a link that has been customized for social media is posted online. Campaigns and organizations fundraising on ActBlue can create social shares for their contribution forms so when they or a donor posts the link to the contribution form, a custom image and text appears. This is what a social share can look like on Twitter for a contribution form created on ActBlue:

Twitter prohibits paid advertising for political content. Anyone can still share a link to political content on Twitter, but they cannot pay for it to be promoted. This includes links to contribution forms for political candidates and causes. Since social shares are not paid for or purchased by any campaign or political organization, they are not political ads, and they are permitted on Twitter.
It’s helpful to note that ads on Twitter say “Promoted” in the bottom left corner. This is what an ad on Twitter can look like:

Not sure if you’re seeing an ad or a social share? You can search Twitter’s ad center.