The Agenda panel allows you to get an overview of your TODOs sorted and categorized on a specific criteria. It's a great place to start your day.
From the agenda you can also add new Todos, update Todos, schedule them and so forth.
The Agenda can more or less be used standalone without ever accessing the editor.
There are several different views in the Agenda panel. For example Agenda view, Calendar view, List view and By Header view.
Each view shows all the Todos which exist in the file with all it information, the state, its priority, the effort, tags, Header and body, including timestamps.
The difference between the views is how they are sorted and presented.
The Agenda view allows you to automatically get a list of the most important things to do today and in the current week. Based on the priorities set on the TODO and any DEADLINE or SCHEDULE, the Agenda view gives a clear overview of what is the most important and/or urgent right now. You clearly see what needs to be done today under the Today heading. Under the Overdue heading is shown what tasks have past their due dates. Under the Important heading are shown TODOs with the highest priority.
You can also sort the items under each heading in the Agenda view by selecting a sort target in the Sort By selector.
Every Headline in the Agenda panel can be clicked to hide/un-hide the included TODOs.
The Calendar view gives a good overview of all scheduled tasks in the current week. Every TODO which has a SCHEDULE and DEADLINE keyword attached to it and where the date falls within one of the days in the current week will be shown.
The Calendar view can also be sorted based on your selected criteria.
The Calendar view is the only view which does not show all the Todos at once, but only the Todos for that particular week.
The List view shows all TODOs one one long list. You can sort this list based on your preferred criteria and you can also filter the list by searching for any text you like.
The list view is the most generic agenda panel view and allows for sorting and searching as you like without any predefined arrangements.
By Header View
The By Header view lets you see the TODOs arrange by the Header they are written below in the text file. This is a good way to organize your text-file/org-file and allows you to track sub-projects within a project.
If you treat your file as one big project, then you can split this project into smaller sub-projects by making headlines with the sub-project name and putting the TODOs under its corresponding headline.
You can sort the TODOs in the Agenda panel by selecting what to sort by in the Sort By selector. If you click the same sort attribute a second time it will sort in the opposite direction.
The search function in the Agenda panel allows you to search on any text in the TODOs. Search is case insensitive, but does not apply fuzzy logic so it finds only exact matches. The search result is presented in the view you already have, by filtering out the TODOs that do not contain the word/sentence you searched on.
In addition to sorting in the agenda view, you can also sort in the Editor view. The difference between searching in the Agenda panel or the Editor panel is that the Agenda panel only searches the TODOs while the Editor panel searches all text, also non-TODO items.
There are several useful features within the Agenda panel and many of those are explained on the Features and Org mode Basics document page. For example, Reveal in Editor, Reveal Parent, promote to Do Today or Do Tomorrow, add/update priority and effort etc.