When to use before insert, before update, after insert and after update in Triggers ? I would like to know in which scenario what to use, since I'm getting confused to use them. Please explain it with an example, so there could be clarity on these.

Update code like below

trigger setProbabilityDefaultOnOpportunities on Opportunity (before insert, before update) {
     for (Opportunity opp : trigger.new) {
         if (opp.probability == NULL ){
            opp.Probability = 10;
         }
     }
}