Das ist eine Tasche.

The indefinite article is used …

• when we are talking about the type of object or person being referred to in general.
• when the object or person is not yet known and has not been mentioned before.

Das ist die Tasche von Jenny.

The definite article is used …

• when talking about a particular object or person that is already known to the speaker or listener.
• when further information is known about a person or an object, e.g. to whom it belongs.