KParts is the name of the component framework for KDE. An individual component is called a KPart. As an example, Konsole is available as a KPart and is used in applications like Konqueror and Kate. Good examples about how KParts can be used are Konqueror, which (among other things) uses the KWord part to display documents, KMPlayer part to play multimedia, and Kontact, which embeds kdepim applications under one roof.
KParts also provide flexibility in your software selection. For example, the KHTML part which is by default used by Konqueror for HTML rendering can very easily be exchanged by a Webkit KPart.