Port Density - this is just a way to describe the number of interfaces. For instance a site that has 24 port switch and the data closet has 100 network drops then the port density needed to accomodate the rest is 76 more ports, or 2 48 Port switches, this will also provide additional ports for future growth.
Throughput - http://en.wikipedia.org/wiki/Throughput - One way to look at this is transmit and receive on a ckt will never be more than the total bandwidth allowed. When looking at an interface on a router you will see the following which tells you how much throughput at a given time.
noc11-br-01>sh int g8/12 | i minute
5 minute input rate 31799000 bits/sec, 3385 packets/sec
5 minute output rate 3167000 bits/sec, 2345 packets/sec
Add the input and output together to give you the total throughput
Switch capacity - as it relates to what? If a switch has 24 ports it has a capacity of 24 ports to be used for connections, however, if you are looking at what type of subscriptions it can do or forwarding then that is a different. You can verify that by checking the data sheets of a given device. What that means is the capability of the switch to process the packets and move them from one end to another. Some do this faster than others, say a 3650 versus a Nexus 7000
