The Inventory is brought up using the 'I' key by default.
On the left hand side are tabs that show certain groups of items:
Along the top there is a filter that can be used to limit the items that are shown in the window to just one class:
A sort method (which can be selected again to reverse the sort order):
And a setting for the view:
Selecting list from this will give more detail on items, without having to mouseover them to show their tooltips (the same view as is used by vendors).
There is little difference between the 'Icons' and 'Tiled' views, other than the size of the icons used for items, and hence the number of columns shown.
In the bottom right your currency in crystite is shown:
And in the bottom left is a trash can, onto which items can be dragged to permanently destroy them:
The white block to the right of the item listing is the drag control for the scroll bar.
Anything that shows up in red is unusable, either through not having the right battleframe equipped, not having it levelled enough, or not having the required materials to make it.