Pencil is a standalone app that lets you create diagrams or UI mockups. It doesn't have collaboration/multi-user features.
Balsamiq Wireframes is a rapid low-fidelity UI wireframing tool that reproduces the experience of sketching on a notepad or whiteboard, but using a computer.
Moqups is an all-in-one online design platform that's smart, simple and fast. Go from diagrams to wireframes and prototypes without switching apps or updating across platforms.