What do these two computer-tech terms mean?

[Alessandro]

http://hashtags.org/
The # symbol, called a hashtag, is used to mark keyword or topic in a Tweet. Any user can categorize or follow topics with the Hashtags service.

A hashtag is a way to link similar content and people who speak the same issues.

Hashtag is a convention that helps the community by adding a context and metadata to your tweets and you create simply by prefixing a word with a hash symbol: # hashtag.

https://en.wikipedia.org/wiki/Retina_display
Retina display is a brand name used by Apple for displays claimed to have pixel density so high that the eye would not be able to notice pixelation at a typical viewing distance.