aside - Typically styled without a title. Similar to a Facebook note update.
gallery - A gallery of images. Post will likely contain a gallery shortcode and will have image attachments.
link - A link to another site. Themes may wish to use the first <a href=””> tag in the post content as the external link for that post. An alternative approach could be if the post consists only of a URL, then that will be the URL and the title (post_title) will be the name attached to the anchor for it.
image - A single image. The first <img /> tag in the post could be considered the image. Alternatively, if the post consists only of a URL, that will be the image URL and the title of the post (post_title) will be the title attribute for the image.
quote - A quotation. Probably will contain a blockquote holding the quote content. Alternatively, the quote may be just the content, with the source/author being the title.
status - A short status update, similar to a Twitter status update.
video - A single video or video playlist. The first <video /> tag or object/embed in the post content could be considered the video. Alternatively, if the post consists only of a URL, that will be the video URL. May also contain the video as an attachment to the post, if video support is enabled on the blog (like via a plugin).
audio - An audio file or playlist. Could be used for Podcasting.