CASA defines an exit seat as any seat that is in a row of seats adjoining an exit.
On the 737-700 and 800, that is any seat in the emergency exit row/s and on DJ aircraft where there is no bulkhead, seats 1ABC.
The following is copied from CASA's website.
CASA does not currently have any regulations, (other than for passengers who are handicapped and for infants), specifically related to who may be seated at an over-wing exit row on an aircraft.
CASA does, however, recognise that a passenger seated in an emergency exit row may be called upon to assist crew members in the unlikely event of an emergency evacuation and provides advice to airlines on the criteria for appropriate passengers to be seated in over-wing exit rows.
This includes:
able-bodied
a minimum of 15 years old
can understand and converse in English
are not travelling with an infant
are not travelling with someone who requires their assistance in an emergency
are willing to provide assistance to cabin crew and other passengers in the event of an emergency.
Cabin crew can move a passenger if they do not meet the criteria and passengers can also asked to be moved to another seat if they are uncomfortable with the requirements of being an over-wing exit row passenger.
CASA is currently drafting new regulations in Part 121, which will contain specific regulations regarding exit rows.
Airlines therefore have their own policies regarding who can/can not be seated in an exit seat.
At Virgin Blue, pax allocated an overwing seat are asked at check-in if they are willing to assist cabin crew in an emergency. If they agree to this and they meet the coy's policy they are then allocated a seat. Once on board the cabin crew will ask a series of scripted questions to confirm that
a) they were told they are sitting in an emergency row;
b) that they are willing to help open exits in the unlikely event of an emergency;
c) they have read the safety instruction card and are familiar with the operation of exit;
d) they are asked if they have any questions.
We do not individually brief the pax in 1ABC.
However, if it is known that there will be an emergency landing, cabin crew will assign ABP's to each exit and a more detailed brief will take place. This includes scenarios for an incapacitated crew member, checking whether or not the exit is safe to use, how to operate the exit (both door and window exits), their features (slides etc) and also how to evacuate safely.
I agree with SydGirl, if you're not briefed and you are sitting in an emergency exit, speak up.
I hope this helps answer some questions.