Assuming you're referring to the "standard" version of Steam, the two major differences from Age of Steam are:
- Split income and VP tracks. When delivering goods, you must choose where your points go, either to an income track (that goes from -10 to +10) or to your victory point track. Early in the game, you'd usually concentrate on income to grow your business, then at some point switch to VPs.
- There is no board with randomized upcoming goods for each city and therefore no "production" action. Instead, 12 sets of 3 goods are available for use with the new "city growth" action or when you place down a new city tile while "urbanizing". A startup city can only get one new set of goods with city growth, and urbanized cities cannot get another set after being placed.
Smaller rule changes include tile costs, tile distribution, no income reduction, loco is not allowed if you're on 6 already, final scoring for tracks is 1 VP per link, and when shipping you can't give more points to any other player.
This great review contains a more detailed list of differences.