New units : apart from the ones mentionned by Dbear you have :
- cruiser
- coracle
- scientific leader
- Modern paratroopers
+ the new civs' UU :
- Maya Javelin thrower
- Inca Chasqui Scout
- Austrian Hussar
- Hittite three-man chariot
- Byzantine Dromon
- Portuguese Carrack
- Dutch Swiss mercenary pike
- Sumerian Enkidu Warrior
+ the units included in PTW such as PTW civs' UU or main game units :
- Medieval Infantry
- Guerrilla
- Arabian Ansar
- Scandinavian berserker
- Spanish Conquistador
- Ottomans Sipahi
- Korean Hwacha
- Mongols keshik
- Celts gallic warrior
- Carthage Lybian mercenary
+ many other scenario units.
Many were included in PTW such as the ones used for the WWII Pacific conquest (zero, Japanese infantry), new ones such as A bomb; guards,voltigeurs, batteries, ship of the line (Napoleonic conquest),...
actually the easiest way to see them all (with pictures) is to check
http://www.civfanatics.com/civ3units_ptw.shtml (PTW units that are either included or that you can download from Civ3 website or at CFC (check in my signature Civ3 stuff, extra-units) and
http://www.civfanatics.com/civ3units_c3c.shtml (for Conquests specific units).
Meaning you don't need PTW as you mostly get all its new stuuf in Conquests or from the web) + all the new stuff from Conquests. Go for it !