Clicking on the Move command, then clicking on a target destination will send the unit you’ve selected to the designated area, ignoring all enemies along the way (even if it is attacked). The Move command should be used very carefully -- if you tell a group of units to move to a location, and enemy units ambush them, they will continue towards the destination rather than responding to the attack. This can quickly lead to a massive slaughter of your forces, if you’re not careful. They will walk by towers, enemy units, or anything else that crosses their path. They will not stop unless they run into a physical barrier (water, trees, or other obstructing terrain) reach their destination, or are given another command.
You should only use this command when you are certain that your enemy will not ambush your forces, or when you don't care if your forces take any damage from any enemy units they meet along the way. (for example, if you're trying to retreat your troops or move past enemy defenses to get to their resources).
Note: right-clicking on an area (not a unit or a building) also issues the Move command.
Clicking on the Stop command will stop all action for the unit you have selected, ignoring and canceling whatever command you’ve issued previously. Stop can be useful when you want to halt a group of moving ranged units (such as marines or stalkers) so that they open fire immediately. If you have a group of marines with Move orders, and they encounter an enemy squad, it's often faster to issue a Stop command than it is to order them to Attack an area, which can come in handy when your forces run into defensive structures or an ambush.
Clicking on the Hold Position command, will cause the unit you have selected to stay put – it will not move to engage enemies in its sight range, even if it is being attacked by ranged fire, though it will attack targets of opportunity with its own ranged attacks. The danger of Hold Position is a simple one -- a single enemy unit with superior range can pick off your troops one by one.
Hold Position can be used to prevent your units from chasing enemy units (air units in particular can follow enemy units into disadvantageous positions). If your enemy baits your units with a single unit to draw them out of a fortified position, you may find that they follow of their own accord if you don’t rely on the Hold Position command.
Hold position is especially useful when you’re besieging a base - if you’re slowly picking your way through a series of defenses one at a time, your enemy can bait your units to rush into a greater force or more automated defense – unless you stop them.
Hold Position vs. Stop
If you don't want your units to chase enemy units that attack them, select Hold Position, not Stop. The Stop command ensures that your units will engage and chase enemies attacking them: if they are Holding Position, they will stand there and suffer damage unless enemy units are in range, but they will not follow if the enemy retreats.
Attack and Attack-Move
You must choose a target after clicking on the Attack command. If you select an enemy unit, your unit will move towards and attack the targeted enemy until it or the enemy dies (or until you issue another command). If you target a location, your unit will move to that destination, attacking any enemy units along the way. When ordering a unit to attack, it is up to you to make sure your forces reach their destination. You will find that picking good "waypoints" and careful control will aid you in this task.
If you select Attack and click on an area, rather than a unit, the forces will "Attack-Move" to the area you clicked on, automatically engaging any enemy units or buildings that come into their range on the way to their destination. They will not react to any units that they are unable to attack.
Attack-Move is by far the safest way to move units from one location to another. If you use the Move command instead, your forces might walk through an enemy ambush, and instead of returning fire, they will continue on while they are being attacked.
Right-Clicking on an enemy unit is the same as selecting the Attack command and clicking on the unit. If, however, you miss the intended target and click on the ground rather instead, you will end up issuing a Move command.
Try to use Attack-Move (via clicking on the surrounding area) rather than targeting an enemy unit, unless you absolutely intend to kill the targeted unit at any cost. When you select a group of units and tell them to attack an enemy unit some distance away, all the units you’ve ordered to attack it will stop moving when the enemy unit is killed. Since you won't receive any warning that they've lost their target, you might not realize that your reinforcements are still sitting halfway across the battlefield.
Clicking on the Patrol command and designating a target location on the map orders the selected unit to move back and forth between the destination and its current position, attacking any hostile creatures that it detects. Units with active Autocast abilities will use them appropriately while on Patrol.
A unit on Patrol will effectively perform an Attack-Move between its location and the point that you tell it to Patrol to. Just like a normal Attack-Move, the unit will automatically engage any enemy troops that it encounters, unless it has no way to attack them (for example, an immortal encountering an air unit).
Once a patrolling unit encounters an enemy, the patroller will attempt to destroy it, pursuing if necessary. The patroller will resume its normal Patrol route once it has dealt with the enemy unless it finds other targets to attack along the way.
Use Patrol with your air units to watch over mineral clusters and vespene geysers, and ensure the enemy does not build on them. Use the Patrol command when you want your units to return to their path after battle or to watch small choke points.
You can place workers on patrol with the repair command active (on ‘autocast’). This will cause them to auto-acquire buildings that need repair and patch up a large area of buildings that are regularly under attack.