To answer your main question "What is Special Defense and Special Attack?", they are 2 of 6 stats of a Pokemon.
To answer your secondary question,"I have always wanted to know the difference between Attack and Special Attack also Defence and Special Defence.":
The difference of Attack and Special Attack are the moves that use these stats. Moves like Tackle uses the Attack stat in calculating damage that your Pokemon GIVES while moves like Water Gun uses Special Attack stat.
It's the same with Defense and Special Defense. Again moves like Tackle uses the Defense stat to calculate the damage that your Pokemon RECEIVES and also with Water Gun, uses Sp.Def
To know if your move will use Sp.Atk or Atk stat, check the description of the move. If it has a red sign beside it, it means it's PHYSICAL therefore uses ATTACK stat. If it has a blue sign then it means it's SPECIAL which uses SPECIAL ATTACK.
TL;DR
They are stats.
Attack and Defense are Physical
Sp.Atk and Sp.Def are Special
Hope I answered your question! :)
