There are 10 user levels in WordPress. See the Codex for more info on those.

The default level for a new user is set on the page
Options > Writing
where the initial permission can be set to

Cannot write articles
May submit drafts for review
May publish articles

corresponding to user levels 0, 1 and 2 respectively.

User levels are reset on the page
Users > Authors & Users

(where an “Author” is any registered user with level > 0). Note that Authors cannot be deleted directly, but they can be demoted to level 0, and then deleted (along with all of their posts).

The permissions for posting comments (as opposed to articles) are somewhat different and are set on the page
Options > Discussion

The ability be commented upon is a property of the article, with the default set on that page. In addition, if comments are allowed, additional options control moderation:

* An administrator must approve the comment (regardless of any matches below)
* Comment author must fill out name and e-mail
* Comment author must have a previously approved comment.

Setting just the last two forces a new user to register and an administrator to approve the user’s first comment — but later ones will go through without moderation.