Glossary

Component:
The components provide the core logic for the functionality in Home Assistant. Like notify provides sending notifications.
Device:
Usually this is a physical unit which can do or observe something.
Discovery:
The automatic setup of zeroconf/mDNS and uPnP devices after they are discovered.
Entity:
An entity is the representation of a single device, unit or web service.
Event:
When somethings happen.
Groups:
Groups are a way to organize your entities into a group.
Platform:
A platform makes the connection to a specific software or hardware platform. The pushbullet platform works with the service from pushbullet.com.
Service:
Services are called to perform actions.
hass:
Often used abbreviation for Home Assistant.