Hi Persefoni,
Kelly already answered your first question in her post.
> Is this what ArcGIS explorer is?
Not quite. Explorer for ArcGIS is a mobile app that allows you to view and access web maps stored in ArcGIS Online. ArcGIS Companion mentioned by Kelly, is an app that enables you to manage and administer your ArcGIS Online organization.
To learn more about Explorer, watch this video: What's New in Explorer for ArcGIS Video | Esri
> Also, I'd like to share a webapp with someone. The only way I know to do that is to email them the URL. Is there some way that they can actually download the app to their phone, instead of having to find the emailed link to follow?
Web apps run within a web browser, so no - you can't enable someone to download it onto their mobile device. Sending them a URL link is the right approach.
Hope this helps,
