Movepool refers to the moves that the Pokemon can learn. Since Sceptile can learn moves such as Dragon Claw, Thunderpunch, Crunch, Earthquake, Aerial Ace, and Brick Break in addition to the very good Leaf Blade (some only through move tutors or breeding), it can be very versatile.
Special attack and Special Defense refer to these types in the games up through 3rd generation (R/S/E/FR/LG/G/S/C/R/B/Y):
Water., Ice, Fire, Grass, Electric, Psychic, Dragon, Dark. The stats determine how much damage a Pokemon can do with them (SPAtk) and how well they withstand them (SPDef). For types not listed, the regular Attack/Defense stats apply. Therefore, use Leaf Blade/Crunch/ThunderPunch/Dragon Claw or sub in an annoyance/support move to take advantage of its SpAtk stat.
D/P have a whole different definition of physical/special attack and defense, not based on type.