Varies a lot. Can depend on airline, aircraft, aircraft config, route, time of day/night, etc.
Main areas of difference (some or all apply):
seating - style, width, pitch, suite/not, recline
IFE - selections available, on demand, screen size, etc
food and drink - on demand, quality, amount of choice, quantity
service - usually less pax per FA in first, quality, attitude
amenities - kit, pajamas, etc
bathrooms - less pax per bathroom in first, size, quality and range of amenities
lounge - first may be separated and higher quality, better food & drink, quieter, more space, etc
baggage allowance - checked and carry-on