Mwk is masterwork, a well-built item - and the explanation for the different attack bonuses. While masterwork items are not magical, they still grant a bonus. In the case of weapons, it's a +1 to attack rolls, resulting in +9/+4 instead of +8/+3.
MWK (Masterwork, add +1 to hit), +9 for first attack on a standard action. Hit with a (1d10+3) for damage (Type will be based on weapon, irrelevant for most PC's or player characters as they lack typed DR). The critical range is next (1d10+3/19-20) 19-20 is a critical threat, confirm to do x2 damage (roll twice add damage bonuses that aren't precision like sneak attacks). so if GM rolls a natural 19 or a natural 20, then confirms over PC's armor class (AC), roll (1d10+3, + 1d10+3) for damage.
Next we have +9/+4. /+4 is the second attack on a full round attack. This can only be used following a +9 attack as part of a full-round action where no other actions (except a 5ft step or a swift action) have been used. Then treat it as a second regular attack on any target in range with identical damages and critical threats.
The /x3 from the (1d10+3/x3) Halberd and long spear represent a critical range of a 20 on a d20, for x3 damage rolled. So a confirmed natural 20 on a roll will grant (1d10+3, + 1d10+3, + 1d10+3). If nothing is listed, it is simply a x2 critical on a d20 critical roll, this is common for most Natural weapons like claw/Bite/Wing/Hoof/Gore/Slam, etc.
An 18-20 critical is just a larger range to confirm a critical. So a natural 18 or 19 is a possible critical, if the confirmation fails the attack fails as normal, on a natural 20 it is a hit regardless of confirmation, confirmation only grants the bonus critical multiplier.
If an NPC has a sneak dice listed it will usually say Sneak attack (+1d6) or such, this is added as per sneak attack rules, it is usually under the weapon block under Special.