There are three networking devices: a hub, a switch, and a router. They operate at Physical, Data access, and Network layers respectively. Nodes connected to a hub are in the same network domain, and in the same collision domain. Nodes connected to a switch are in the same network domain, but not in the same collision domain. Nodes connected to a router are in different network, and different collision domains. So, a switched network will protect you from collision but not from network attacks.
Collision means that packets sent from two different nodes may cancel or interfere with each other. For example, Andy sends a packet to Cornius at the same time that Bill sends a packet to Danny and these two packets may collide with each other, physically. In other words, they share the same medium.
From a security perspective, being protected from collision means that you are protected from packet sniffers (there are ways around this), and you can enjoy better bandwidth.