Is past participle and simple past the same? If not, why?
The past participle is used as part of a "verb phrase" to form the past perfect tense (that is a tense that indicates a action was completed before some other event).
Example of use of past participle in past perfect tense:
The boy had looked everywhere for his homework when the dog came into the room with it in his mouth.
Compare this to the simple past:
The dog looked at him apologetically.
Note that for some verbs the past particle and the simple past do not have the same form.
Example: simple past.
Joe ate his breakfast.
Example: past particle
Joe had eaten his breakfast before his mother came into the kitchen.
Also note that the past particle may also be used in the passive voice or as an adjective (but I don't think giving further examples of these would add much to this answer).