My opponent (we're both beginner players) casts a Renegade Freighter, which is an Artifact — Vehicle. Can he attack me with it on his next turn?

Yes, a creature can attack when it has been in play, under the same players control since the beginning of the turn. ( Or if a creature has haste  )

Also, it says "Crew 2", which, when enabled, turns it into an artifact creature. What is an artifact creature?

Crew X is an ability, when activated adds creature to its types.

So the card by default reads:

Card Name: Renegade Freighter

Converted Mana Cost: 3

Types: Artifact — Vehicle

Card Text: Whenever Renegade Freighter attacks, it gets +1/+1 and gains trample until end of turn. Crew 2 (Tap any number of creatures you control with total power 2 or more: This Vehicle becomes an artifact creature until end of turn.)

P/T: 4 / 3

After a player activates Crew 2 the card reads:

Card Name: Renegade Freighter

Converted Mana Cost: 3

Types: Artifact — Vehicle Creature

Card Text: Whenever Renegade Freighter attacks, it gets +1/+1 and gains trample until end of turn. Crew 2 (Tap any number of creatures you control with total power 2 or more: This Vehicle becomes an artifact creature until end of turn.)

P/T: 4 / 3

This makes the card a creature, so if you have a card that says Kill Target Creature, you are now able to target Renegade Freighter.

My opponent (we're both beginner players) casts a Renegade Freighter, which is an Artifact — Vehicle. Can he attack me with it on his next turn?

Yes, if it's a creature. A creature can attack when it has been in play, under the same players control since the beginning of the turn (or if it has haste), even if it wasn't a creature previously.

Also, it says "Crew 2", which, when enabled, turns it into an artifact creature. What is an artifact creature?

Crew X is an ability, when activated adds creature to its types.

So the card by default reads:

Card Name: Renegade Freighter

Types: Artifact — Vehicle

Card Text: Whenever Renegade Freighter attacks, it gets +1/+1 and gains trample until end of turn. Crew 2 (Tap any number of creatures you control with total power 2 or more: This Vehicle becomes an artifact creature until end of turn.)

P/T: 4 / 3

After a player activates Crew 2 the card reads:

Card Name: Renegade Freighter

Types: Artifact — Vehicle Creature

Card Text: Whenever Renegade Freighter attacks, it gets +1/+1 and gains trample until end of turn. Crew 2 (Tap any number of creatures you control with total power 2 or more: This Vehicle becomes an artifact creature until end of turn.)

P/T: 4 / 3

This makes the card a creature, so if you have a card that says Kill Target Creature, you are now able to target Renegade Freighter.