Egg Groups in Pokemon Platinum and all other Pokemon games are what determines what Pokemon can breed. For example a male Shinx can breed with a female Ziggagoon cause they are both in the same Egg Group which is Field. Some Pokemon can be in up to two Egg Groups for example Ekans is in the Field and Dragon Egg Groups.
You have to leave two compatible pokémon (a male and female of the same species or in the same egg group) at the Day Care in Solaceon Town.
Yes, if it is in the egg group or is a ditto.
the female is the same breed as the egg
by trading or cheating with an action replay-if you already have one you can also breed with a Pokemon of the same egg group to get another
Normal egg groups, same as Diamond, Pearl, Platinum, HeartGold, and SoulSilver.
you have to talk to the day care man and if he says "the two seem to get along" it means that they are in the same egg group
Clefairy's Egg Group is Fairy, so any Pokemon with the same Egg Group can breed with Clefairy, including Ditto, which has its own Egg Group.
you get it from an egg in Pokemon platinum
To get eggs, you must have two Pokemon of the same species and opposite gender. Put them both in the daycare in Solacean Town at the same time. Eventually, they will lay an egg.
no you can't find a torchic egg in Pokemon platinum
You have to breed an electabuzz. Find an electabuzz on route 222 (the bit between the hotel and Sunyshore)and breed it with a ditto if its male, or if its female, a ditto or another Pokemon from the same egg group.
Egg groups indicate which Pokemon can breed with which. One Pokemon can be in several different egg groups, and some are in no egg groups at all because they cannot breed (legendary Pokemon, for example). If you want to breed your Pokemon, first look up what egg group(s) it is in, and then what other Pokemon are in the same group(s).