Here's why:

* Colony: A visible group of bacteria that originates from a single bacterium, multiplying and forming a distinct mass on a solid growth medium.

* Biofilm: A complex community of bacteria that adheres to a surface, forming a slimy layer. Biofilms can involve multiple species of bacteria.

While both terms involve groups of bacteria, colonies specifically refer to the visible, individual clumps formed by bacterial growth on a surface, while biofilms are more complex and involve a community of bacteria interacting within a matrix of their own secretions.