Endpoint is "the daemon, worker, or service that a client communicates with to access an API. In OpenStack, API endpoints can provide services such as authentication, adding images, booting virtual machines, and attaching volumes." In other words endpoint is an URL where you must send requests to manage you OpenStack. Each service has its own endpoint.

Namespace in OpenStack is a network-related term. You can find its description here: http://docs.openstack.org/grizzly/openstack-network/admin/content/adv_cfg_l3_agent_namespace.html

Instance is in fact a virtual machine and node is usually refers to a server (bare-metal), which is used for different OpenStack services (i.e. compute node is used for managing VMs (instances) and contains Nova-related services, controller node is used for managing all the components of OpenStack and can contain such components as DB, identity service, Horizon UI, etc.). In fact one node can contain several different services.

Also I suggest you to look at OpenStack Glossary: http://docs.openstack.org/trunk/openstack-ops/content/go01.html and use search form at this page: http://docs.openstack.org/

And you can find very useful articles, which will help you understand what does OpenStack consist of here: http://docs.openstack.org/grizzly/openstack-compute/admin/content/ch_getting-started-with-openstack.html