IF and IW are very similar roles which are a common choice for many Football Manager tactics. The IF role in particular is quite popular.
IF stands for Inside Forward.
IW stands for Inverted Winger.
The IF role can be set only at the AM RL positions, while the IW role can be set at both AM RL and M RL positions.
The key difference between the two roles is that the IF is a bit more of a forward, whereas the IW is a bit more of a midfielder.
Therefore, the IF generally looks to be more involved in the finishing stage of attack, while the IW tends to be more involved in the build-up phase.
But apart from these couple nuances, their general behavior is largely similar. And the vast majority of players who can play the IF role can equally successfully play as an IW as well.
Source: SI Forums