@joao the Friends category is a place for you to follow the people and things you care about most. Noise is a place for everything else. Friends is for considering things closely. Noise is for more of a surface glance. But, really they're both just part of a simple duality, offering two ways of looking at things. Use them however you'd like. The people you follow don't know which category they're in, just that you're following them.
Shift + 5 will allow you to toggle the linear Friends feed to a fluid grid, and vice versa, to toggle the fluid grid of the Noise feed to a linear feed with larger bits of content.