There are different terms for the positions based where your from and tactical method your playing. In general using a 4-3-3 there are:
Forward(s) you might play with just 1 or up to 4 in this case 3. One up top and left forward and right forward - Their role is to execute goal scoring opportunities. Their positions overlap each other to make angles and space. No strict rules on positions at the top 9as long as they work in sync).
Midfielders - Deliver mail is their role. they create scoring opportunities. They are the transition between the back defensive half and the offensive half. I call it delivering mail..Typically most creative and with most vision do best here. One center midfield and two wings consist of midfield (wings are runners and can keep high pace)
Defense- role is to eliminate opposing teams offensive play by pressuring and regaining possession. These four players are:
Left back Right back Stopper ( in between left and right back typically helps midfield as well)
Sweeper - Leader...verbal and loud manages the defensive half of field. Decides when to move line up for off sides trap. Sweeper is the last man in defense.
Hope this helps...this is more tactics but the mindset of the positions is the important part.