From Liquipedia Starcraft 2 Wiki
Force Field is an offensive/defensive spell cast by the Protoss Sentry. When cast, the temporary Force Field will block the movement of any ground unit (Massive non-hallucination units excluded). Each Force Field lasts for 15 seconds.
In competitive play Force Field is often used to channel enemy forces into an area or in a direction that is beneficial to the Protoss player. Multiple Force Fields can block a ramp, preventing an early rush from entering the Protoss player's base for a short period of time. Force Field can also be used offensively, as it allows the Protoss player to divide portions of the enemy army, making it much easier to deal with the enemy in pieces.
The effective use of Force Field can be divided into Choke Play and Open Field Play.
 Choke Play
Defensive Choke Blocking: This is when you use Force Field to defensively block a choke, often your own ramp. It's useful to do this when you don't have enough units to fight the enemy. If you can sense the enemy moving in, you Force Field a choke to prevent them from entering. If you time this Force Field right, you can actually split your enemies forces in half, trapping some of them near your forces where you can kill them. Alternatively if they plan to retreat while fighting you, you can force field a choke area to force them to fight you.
Offensive Choke Blocking: This is when you use Force Field to block your opponent's choke. It's useful to do this when you're attacking your opponent's natural expansion. Force Fielding your opponents ramp prevents reinforcements from arriving from his main base, and prevents the opponent from retreating any forces he has at his natural back up into his main. You can also offensively block your opponent's ramp while harassing with a Warp Prism to prevent your opponent from moving his forces between bases, blocking him from dealing with your harass. These tactics aren't limited to ramps; they can be used on any kind of choke, but using them on ramps is the most typical application.
 Open Field Play
Protecting Flanks: You use Force Field to block your army's flank. It prevents your opponent from easily getting into your backfield and limits your opponent's ability to surround you. This can create a nice choke for your Zealots.
Dividing the Opponent's Army: By dividing your opponent's army at odd angles with Force Fields you can take advantage of his lack of coordination and poor ability to focus fire. This technique can also force an engagement if it manages to trap some of the opponent's forces.
Channeling the Opponent's Army: You can force your opponent through a small area, making it difficult for him to get a good arc and giving you a positional advantage.
Blocking a Retreat Path: This is just putting a Force Field in your opponent's retreat path. This can be still be effective with as little as a single Force Field in open terrain if placed directly in a units retreat path.
Preventing the opponent from kiting" By Force Fielding behind enemy units, Zealots will be able to close and deal massive amounts of damage. The enemy units will not be able to kite the Zealots and must take maximum damage from the Zealots.
 Vs. Protoss
In the early stages of this match-up, where Colossi are not likely to have popped out, Force Fields are extremely powerful for both defensive and offensive tactics. Force Fields are extremely powerful against the 4Gate thanks to the fact that only one Force Field is needed to cut the opponent's army into small bits and pieces. During an attack or confrontation in the middle of the map, Force Fields will benefit Zealots extremely well by preventing Stalkers from Kiting.
Force Fields after the early game become less useful as more Massive units make it to the fray such as Archons and Colossi. Blink Stalkers will also mitigate the power of Force Fields by ignoring positional disadvantages.
 Vs. Terran
When fighting against a Terran opponent, Force Fields are essential for early game to late mid-game defense. Because of Concussive Shells and Stim, Zealots will be unable to hit MMM very much since Marines and Marauders will Kite them forever. Because Zealots are needed to kill off pushes, Sentries need to assist the Zealots by Force Fielding behind MMM forces in order for Zealots to deal significant damage.
When defending bases, Force Field timings will depend on your unit composition. When defending with mostly Zealots, force Field BEHIND most of the Terran forces. With a more Stalker heavier composition, split MMM forces in half.
 Vs. Zerg
Force Fields are extremely powerful for protecting bases and units. Combined with a good building placement, Force Fields will prevent Zergling from flooding a base and Roaches from getting past defenses.
Against masses of Zerglings, Force Fields can be employed to save fragile ranged units like Stalkers, Immortals, and Sentries. By surrounding one's small army while leaving small chokes for Zealots to engage, a Protoss player can create a temporary "bunker" where the Zerglings cannot attack.
Roaches suffer more from Force Fields than other ranged units due to their short range of 4. Roaches cut off from a Force Field wall are extremely unlikely to shoot anything past the wall, making Sentries much more devastating to Roaches than they appear. Force Fields can also be used to prevent Roaches with Tunneling Claws from burrowing with the addition of more Force Field walls.
Combined with units like Zealots, Archons, and Colossi, Force Fields will magnify the damage output of those units by forcing ranged units like Roaches and Hydras into horrible positions where they are vulnerable to Zealots, Archon Splash, and Colossi Splash.
 In Combination with Other Spells and Abilities
- When combined with Guardian Shield, well placed Force Fields can allow a Protoss army to survive and even win battles in which they had an initial disadvantage. By dividing and blocking enemy armies, the Protoss player essentially has the ability to make their opponent play by their rules, and the Guardian Shield will allow them to survive long enough to see that plan come into fruition.
- When combined with Psionic Storm a couple of Force Fields greatly restrict enemy movement even in an open field, greatly increasing the odds that Psionic storm will deal its full damage. By completely walling an enemy in, or even forcing them to escape through narrow checkpoints can allow you to defeat a much more powerful force, or a smaller force with fewer losses.
- Will push units out of the way should it be cast where units stand.
- As of Patch 13 massive units can destroy Force Fields by touching them. This rule doesn't apply to hallucinations of massive units.
- Prevents Terran buildings from landing.