This page explains very briefly how posts work.
Posts are actually the pictures uploaded to the site. Some users use "post" in different contexts, such as with forum messages, but, strictly speaking, posts only refer to the pictures.
It's okay to call posts "pictures" or "images," as long as you get your point across.
Searching a post
There's a textbox labeled "Search" on the side when viewing posts, where you can type in search queries.
The workings of that textbox are explained in the help:cheatsheet page.
When viewing a post or the main page, there is a listing of tags on the side, underneath the search bar.
Those tags are usually preceded by a ? (question mark), which signifies that the tag does not have a wiki page - only a few, more common, tags have wiki pages.
Clicking on it, you will be redirected to the wiki page for that tag.
That page usually explains when it's appropriate to use it (especially for yukkuri-only terms which are never used in other booru sites.)
The number after the tag is supposed to show the number of posts with that tag, but, due to some technical issues, the number is inaccurate.
Some users have a "Mode" drop-down menu below the search box.
The most notable options are "Edit" and "Tag script".
The first lets a user edit a post without leaving the main page, while the second allows a user to tag multiple posts without having to navigate to the specific post each time.
For example, using the "marisa -reimu" tag script, each selected post will be tagged with "marisa", and have any "reimu" tag removed.
Some posts have borders of different colors:
The post is waiting to be approved. This means that an administrator has to accept the post before it is considered a "real" post.
New users can't upload without putting the post in the waiting list.
As long as the uploaded content follows the rules, the blue border is just a formality.
This post is a parent of another post. When viewing it, a bar showing the child(ren) post(s) is displayed.
This post is a child of another post. When viewing it, a bar showing the parent post is displayed.