Thanks Mister Micawber for your quick response.
What I did understand from your answer was this:
inhibits means stops or limits platelet's grouping themselves
And
irreversible means the platelets will never recover their grouping ability
So, I conculded that reversible could mean that they could recover and get back to aggregation process but irreversible means that it only will go to one direction of the process which is disable the aggregation of the platelets which causes thrombus, however, excessive uses of Aspirin could lead to bleeding disorders.
Does my conclusion is right?