There's a very big difference between storage of relational/row-oriented databases and column-oriented databases. For example, if I have a table of 'users' and I need to store friendships between these users... In a relational database my design is something like:
Table: users(pkey = userid) Table: friendships(userid,friendid,...) which contains one (or maybe two depending on how it's impelemented) row for each friendship.
In order to lookup a given users friend, SELECT * FROM friendships WHERE userid = 'myid';
The cost of this relational query continues to increase as a user adds more friends. You also begin to have practical limits. If I have millions of users, each with many thousands of potential friends, the size of these indexes grow exponentially and things get nasty quickly. Rather than friendships, imagine I'm storing activity logs of actions taken by users.
In a column-oriented database these things scale continuously with minimal difference between 10 users and 10,000,000 users, 10 friendships and 10,000 friendships.
Rather than a friendships table, you could just have a friendships column family in the users table. Each column in that family would contain the ID of a friend. The value could store anything else you would have stored in the friendships table in the relational model. As column families are stored together/sequentially on a per-row basis, reading a user with 1 friend versus a user with 10,000 friends is virtually the same. The biggest difference is just in the shipping of this information across the network which is unavoidable. In this system a user could have 10,000,000 friends. In a relational database the size of the friendship table would grow massively and the indexes would be out of control.