Boards
Boards visualize the state of requirements or work on a project. Columns correspond to stages of the process, production stages, or states. Moving a card from one column to another changes its state. For example, when design is finished, you can move a requirement card to the "Development" state.
The board is divided into horizontal sections (swimlanes) that group cards by a chosen property. For example, cards can be grouped by assignee, release, sprint, and so on. Cards can be moved between groups, which assigns them to participants, changes the planned sprint, and so on.
The first column (Backlog) is not divided into groups, because it is the log of every available requirement or task. Requirements wait here until they are planned and assigned to a release or iteration.
Because there can be a great many cards, it helps to collapse them. They then take less space, while you can still see how many cards there are and how they are distributed, for example by priority. You can collapse a single cell with the button in the bottom-right corner of the cell. You can collapse a whole column: open the context menu on the column name and choose "Collapse cards".
You can collapse the cards on the entire board, or simply control the scale (size) of the cards, with the slider in the top-right corner, next to the green buttons.
The switch next to it shows or hides dependencies between cards. This visualizes links on the board, such as blocks and other kinds of dependencies, so you can plan work on the cards with those dependencies in mind.
To add or remove a column (state), open the context menu on the column name and choose an action, for example "Add column to the right". The same menu can edit transitions between states. Full information about states and transitions is on the project settings page.
To configure which attributes appear on cards, and to set grouping, sorting, and the card color scheme, use the gear icon in the top-left corner of the board.