Straight from page 26 of the rulebook (latest downloaded version):
Strike First: A warrior with a spear strikes first in the first turn of hand-to-hand combat
Unwieldy: A warrior with a spear may only use a shield or a buckler in his other hand. He may not use a second weapon.
Cavalry Bonus: If using the rules for mounted models, a mounted warrior armed with a spear receives a +1 Strength bonus when he charges. This bonus only applies for that turn.
So, to answer you question, a spear is a one-handed weapon that cannot be used with any other weapons (as per the Unwieldy rule).
Also remember that as per the Rules Review, Strike First only means that it strikes in Initiative order against a model that also has the strike first rule (i.e. a charging model). This has made spears somewhat pointless on the majority of low-average Initiative models.