performance testers do load testing, stress testing etc.
functional testers  test only functionality.

Not only the functional area and Usability testing also
includes in functional testing.Performance testing comes
under Non-Functional testing

Functional testers concentrate on the functionality of the 
application.whether it is working fine or not as per the 
specifications. generally these functional testers are also 
known as Black-box testers.

where as performance testers concentrate on the performace 
of the application such as response time,throughput.
these performance testers test the performance of the 
application by using some performance tools such as Load 
runner,jmetre..

Well Guys its a very common interview question :

Functional testing : •It is testing of application 
functionality and examination of its compliance with the 
software requirements specification (SRS).

Usability Testing: Lets learn this with an example

E.G If you go in a hotel where there are 5 rooms
Room 1 - Bed room
Room 2 - Kitchen
Room 3 - Bathroom 
Room 4 - Noraml room 
Room 5 - Servent room

If you are asked to go in kitchen what will you do .. you 
open each room to check which one is the Kitchen.
if we need to make this task simpler what we need to do ?
we just put a lable in front of each room then what I need 
to do just read the label and find where the kitchen is..
this is called usability testing . 
In short the button that is label as resgistration form 
opens the resgistration form.

Perforamance testing :

Well performance testing is a mixture of two things

Load Testing , Stess Testing.


Now what are they
Load Testing : E.g If 100 user hits on a same button on a 
website at same time it is called load testing.

Stress Testing: E.G If 100 user hits on 6 different buttons 
on same time is called sterss testing