Summary: This is used for the summary text when showing a list of articles. There is a maximum of 1024 characters, although in practice it is wise to limit it to about 500 characters. The character count on the side of the text box changes color to red if you exceed 500 characters. For most applications, and using an out-of-the-box template provided by Silver Earth, this should not include html – but you can change this policy depending on design decisions made by your developer.
Title Attribute: If specified this will be used for the Title attribute on an image in places where a summary description is not shown. Out-of-the-box, if a title attribute is not specified, then a summary description entered above is used instead. Title attributes, unlike other html, for most browsers is displayed with carriage returns. So include regular carriage returns where you want to break a line for easier readability. Max characters is 1024, although in practice 300 or 400 characters is enough.
This definitely should not contain any html or quotes because it is places as an attribute in an html tag. If you make a decision to only allow the title attribute to be used if specifically entered on your website, then it is safe to include html in the summary description.
Meta-Description: This is used both for your meta-description in the html for your article page, as well as for the open-graph protocol description tag used by Facebook and other social media. Max characters is 1024, but usually only the first 250 or so are used by most search engines.