Sending is time spent uploading the data/request to the server. It occurs between blocking and waiting. For example, if I post back an ASPX page this would indicate the amount of time it took to upload the request (including the values of the forms and the session state) back to the ASP server.
Waiting is the time after the request has been sent, but before a response from the server has been received. Basically this is the time spent waiting for a response from the server.
Receiving is the time spent downloading the response from the server.
Blocking is the amount of time between the UI thread starting the request and the HTTP GET request getting onto the wire.
The order these occur in is:
*Blocking and DNS Lookup might be swapped.
The network tab does not indicate time spent processing.
If you have long blocking times then the machine running the browser is running slowly. You can fix this by upgrading the machine (more RAM, faster processor, etc.) or by reducing its work load (turn off services you do not need, closing programs, etc.).
Long wait times indicate that your server is taking a long time to respond to requests. This either means:
The request takes a long time to process (like if you are pulling a large amount of data from the database, large amounts of data need to be sorted, or a file has to be found on an HDD which needs to spin up).
Your server is receiving too many requests to handle all requests in a reasonable amount of time (it might take .02 seconds to process a request, but when you have 1000 requests there will be a noticeable delay).
The two issues are related. If you can reduce the work load on the server by caching, adding new server, and reducing the work required for active pages then you should see improvements in both areas.