There are 15 positions on the field for each team, each given a specific jersey number. The positions are separated into forwards and backs.



Forwards:
#1: Loosehead prop
#2: Hooker, rake
#3: Tighthead prop
#4 and 5: Locks (aka Second Rows)
#6 and 7: Flankers, wing forwards
#8: Number eight, eight-man
Front row = props and hooker
Second row = locks
Back row = flankers and 8-man

Backs:
#9: Scrum-half, inside half, half-back, scrummie
#10: Fly-half, fly, first five, five-eighth
#11: Inside center, first center, second five-eighth
#12: Outside center, second center
#13 and 14: Wings, wingers
#15 Fullback, sweeper, custodian
Halfbacks = scrumhalf and flyhalf

Substitute players are usually numbered #16-22

For more information, go to Wikipedia