Certain tools on the Moodle front page as well as the Course pages can occur in the form of blocks. These are located in the side bars to the page. The number of blocks available is considerable, but no course will have all of them active. The following blocks are the most likely to occur in a course:
This block gives an overview of all the participants in a course with some of their particulars. You can see more if you click on someone's name or user picture. The participants page can be used to send a message to all or a selected number of users in the course. The functionality of the People block is now included in the Online Users block, so in courses that use the latter, the People block can be hidden.
The Messenger block allows users to send each other messages in Moodle. It is not necessary for users to be in the same course.
This block shows other active users in the course, which is useful if you want to chat or send them a message (click the envelope icon behind their name to open the messenger). At the bottom of the Online Users block is a link to the participants page (same as in the People block), which can be used to send a message to all or a selected number of users in the course.
This block gives an overview of the different resources and activity modules used in the course with their descriptions. For example, clicking "assignments" will list all (visible) assignment in the course, with their deadlines and grades (if any).
In courses with many forum discussions the Search block can help you locate messages that contain certain words or phrases. In courses without much forum discussion this block is often hidden to save space.
The admin block contains for students a link to their grades page, which lists the grades obtained for different activities and in some cases short feedback. There is also a direct link to your profile in the Admin block.
This block lists the courses to which you have access and links to the overview of course categories and courses in which you may enrol (provided you have the enrolment key).
The Latest News block lists the most recent messages posted by the teacher(s) of a course in the News forum (clicking on the more link takes you to the full message). This block is often hidden, because teachers do not post messages in the News forum that often and News messages are flagged on the course pages itself anyway.
The upcoming events block calls attention to entries in the course calendar, like upcoming assignment deadlines. It also links directly to your Calendar.
The Calendar block shows activities and deadlines related to the course it is in. The Calendar block on your MyMoodle page shows entries from all your courses.
The recent activity block gives an overview of what users have done in the course since your last login, for example who wrote forum messages, or who edited wiki pages and when.
Blocks that are not present by default
Blog Menu and Blog Tags
These blocks belong on the Blog pages, but they may be added to a course if the blog function of Moodle is used as part of the course activities.
This block is most common on tag pages, but it can also appear in courses.
The Campus Email block gives access to a course internal email system. The Campus Email block may be added to courses that rely on private messages a lot.
Random Glossary Entry
In some language courses you might come across one of these. It lists another word from a course glossary every time you reload the course page. The link at the bottom takes you to the glossary. For glossaries in which you are allowed to add entries there will be a second link for that purpose.
This is a third party block that allows users to search for terms in Wikipedia (in different languages).
There are a few more blocks that are very rarely used.
Note that it depends on the course and the teacher which blocks are actually displayed in a course. Even standard blocks can be hidden from view and sometimes the order of the blocks differs from course to course. Depending on the theme used, course users can collapse blocks on their course page, so as to save space (but do not collapse blocks that you need to keep informed about the goings on in the course!)