As I has already introduced about MD5 that explained about what MD5 is? its history with pros and cons, then you can see that MD5 is non-reversible. So why you still see MD5 Decrypter on the internet?
How MD5 decrypter works?
They (MD5 decrypters) are not really doing any reversal with your given MD5 String.
They actually register all kind of string that users have entered and the MD5 output into a database, then, when user take a look a MD5 hash decrypter, they look up in the database to see if there are any recored with that MD5 hash.
So it should be called MD5 Lookup.
Then with those explanation, you can see that all kind of MD5 Decrypters only work for some common phrases or you are betting on your luck.