Is there any actual difference between a topic and a service?

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http://wiki.ros.org/ROS/Technical%20O...

The above link quotes that:

Establishing a topic connection: Putting it all together, the sequence by which two nodes begin exchanging messages is:

1. Subscriber starts. It reads its command-line remapping arguments to resolve which topic name it will use. (Remapping Arguments)
2. Publisher starts. It reads its command-line remapping arguments to resolve which topic name it will use. (Remapping Arguments)
3. Subscriber registers with the Master. (XMLRPC)
4. Publisher registers with the Master. (XMLRPC)
5. Master informs Subscriber of new Publisher. (XMLRPC)
6. Subscriber contacts Publisher to request a topic connection and negotiate the transport protocol. (XMLRPC)
7. Publisher sends Subscriber the settings for the selected transport protocol. (XMLRPC)
8. Subscriber connects to Publisher using the selected transport protocol. (TCPROS, etc...)

In the above mentioned steps , the subscriber is requesting for the connection first and then the publisher is sending the settings for the protocol . So isn't it a two way here in topics as well . Where does the exact difference between a topic and a service comes from ?