There are two primary columns: the board is shown on the left and all the other controls on the right. The main means of interaction is dragging and dropping to-do cards into the various lists. The board structure is very customizable, and includes a variety of features that help along the way: co...
GitLab is a Git source control server (like GitHub but for private repositories), and a GitLab CI for builds. Both and more integrate with the issue tracker. For example submitting a commit with a bug ID in its scription will update the bug.
While a ticket can have multiple comments added to it, and (separately) also attachments — if the users have the right permissions — it is impossible to edit a comment after submitting it. This is probably why a comment must be previewed before it can be submitted.
Some of the bigger missing features are integrations with other systems. More reports, custom dashboards and importing and exporting of work items. For those that do not use these features it may not be a large issue, but for those that do, it could really limit the usefulness of this tool.