You can view the cards in your deck in a variety of arrangements. For example, each card can be displayed individually or sorted by mana cost.
While deck building or viewing the details of a deck, there are two options for changing the way cards are arranged: grouping and arranging.
The 'Group by' drop down menu determines whether your cards are displayed individually or in a group and how they are grouped.
Selecting None will display each card individually.
Selecting Printing will group cards with the same name and from the same set together.
Selecting Name will group cards with the same name together, regardless of which set they're from.
The 'Arrange by' drop down menu determines the categories your cards are sorted into.
Selecting Card Type will sort cards into categories based on their card type (creatures, lands, sorceries etc).
Selecting Mana Cost will sort cards into categories based on their mana cost (1 drops, 2 drops etc).