base.html
<!DOCTYPE html>
<html>
<head>
<title>My Project</title>
</head>
<body>
{% block content %}{% endblock content %}
</body>
</html>
And say you have an app called myapp with a view.html page,
{% extends "base.html" %}
{% block content %}
<h2>Content for My App</h2>
<p>Stuff etc etc.</p>
{% endblock content %}