Please Explain post allert procedure

I am just getting into tournament bridge and find the rules a bit daunting.

If I understand post alert requirements correctly, no alerts are made
after 
3N or greater has been bid until the auction is complete. Offense alerts 
before the lead card is played, defense alerts after the lead card is 
played. An example of a post alert would be people playing Gerber (4c)
with 
an agreed to suit (rather than NT). Is this correct? What other common 
bidding conventions require a post alert (i.e.. splinters)? What do you do

if the opposition asks what a bid means, do you tell them to wait until
the 
auction is complete?