Backpack, Bank, House safe and Clan Banks are all inventories where you can store your stuff. You can put containers (Bags, Sacks, Boxes), inside the formerly mentioned inventories which can be used to sort your items as there are no grids. Most containers come in various colours and all containers can be named by clicking them and then pressing backspace.
To split item stack hold the SHIFT key while dragging the item.
Holding the SHIFT key while dragging a non-stackable item which you have multiple of in your inventory will move all those items into a nice pile.
Your backpack, which is your character inventory, shows the weight status on top and bottom of the backpack window.
It is possible to go over the maximum weight but at that point you will move very slowly.
At the bottom right corner you will see the amount of gold in your global wallet. You can add to it in a bank window.