Collecting Minerals
The one of the most important factors in any strategy is the collection of resources. The collection of these resources should be as smooth and as quickly as possible. When collecting resources in Starcraft you have to take into consideration the way resource gathering works in order to build up on economy in the most efficient manner.

When mining mineral clusters you will notice that only one drone is allowed to gather from a single patch, everyone else has to wait for it to finish before they can get a chance to mine that same patch. With this in mind, you can see there will be a certain limit on the amount of drones you will be able to build before it starts to get a little bit crowded.

Keeping in mind that you can only have one drone mine a mineral patch at a time, there is also a distance of time where the drone returns what it has gathered to the hatchery. During this time the mineral patch it has mined is free for another to use. The time that the drone is gone another drone is able to gather resources and return it to the base at about the same time the first drone makes his way back.

Now that those two concepts are out the way, you can start to see that 2 drones are able to mine one crystal by means of playing tag. Now lets take into consideration the entire mineral field. If there were 8 mineral patches you will be able to put 16 drones in order to get the most effective means of gathering resources.

Vespene gas
Gathering vespene gas works in along the same lines as the tag/relay format. Only one drone is allowed in the mine at one time. When it goes to return another is able to get in. On average 3 or 4 is the highest amount allowed before it starts to get crowded. Note that this situation is only for starting locations, where your hatchery is in a fixed location. The farther the location of the gas mine the more drones allowed in.

Expanding
Expansion is important in extended games of sc. Main because you will need to expand to support what I like to call hi-tech units or units that are higher in the tech tree like ultralisk, battlecrusiers, and carriers. The key to expanding is to first make sure that you have enough minerals to create another base without it having to be a heavy burdon on your resources.

There is no set timing on expaning. Basicly its all in judgment and the type of things you would want to do. If you are going to start building up on high-tech units then it would be good to expand to a few extra areas so that you will able to build them without worry of running low on resources.

Another important note is that each expansion should always have an amount of workers relative to the amount of resources. ( See blue notes above)

Offense
Now this is where the real strategies start to come in. When playing on the offensive, head on attacks will not always work. By this I mean taking and just attacking at random against another right in to the line of fire. Instead try to find ways to sneak up on your opponent or counter thier defenses. Take for instance that the player has only melee units, attacking by air would be the best approach.

Keeping your units varied is also important, esp. if you are using Terran and Protoss. If you build various units you will be prepared for all types of situations. With both melee and distance attackers you can cause a lot more damage to your opponents units than if you had melee units only. If you have all melee units, there are only a few that can actualy hit other units before it starts to be crowded and any other melee units you may have will have to wait for thier turn, but if you had both melee and distance units the melee units can hit the enemy up close while the distance units hit them from a distance, which basicly gives you twice the hit ratio.

It also a good idea to keep your attacks varied. It is not always a good idea to keep attacking your opponents in the same spot all the time since they will eventualy defend that area a lot more heavily. Instead attack at one point and then attack at another later on. This way they will not know which side or angle you will be coming from and it is almost always a surprise attack.

Unit control is also important for offense, when you have large groups of units the best thing to do, so that you can quickly control then without having to constantly select every unit, use the control # command. Just select a group, hold cntrl and press any #, to save that group selection. Whenever you press that number it will recall all selected units.

Also be aware that the {patrol} command isnt always the best thing to do. While patrol is very effective, it isnt always perfect. When you're attacking, sometimes there will be large damaging units along with back-up units. While in this situation you have to take into effect which one will cause the most damage to you. If there are 7 marines, but also 3 siege tanks, you should first attack the siege tanks and disreguard the marines until later. If you had partol on, then your units would have hit the closest units first, or the marines, allowing the 3 siege tanks to deal a great amount of damage to your army.

Defense
It is said that the best offense is a good defense. This can be true in some ways. If you use your defenses wisely, they can greatly affect the outcome of the game. Lets say that a player attacks you and all his units have been killed off. If you have units left, make a counter attack since it is at this point that most players will be vulnerable.

When setting up a defense you want to guard mainly your resources first since they are the most important.

When building your defense you must take into consideration the type of map you are playing, which is very obvious. If you are playing island maps then naturaly you would build more air defenses and on land you would get more land defense. Also take into consideration that you should build defenses for both land and air on any map to be prepared for certain situations where a player will go for air on land maps or the other way around.

When building up a defense you do not want to put all your time and resources in to it. You want to be able to find a cheap and also effective way to kill off any attacks. A good example of this would be on maps that have a narrow opening to your base. You would want to put defenses at that key entrance to your base so that the enemy doesnt break through. Also note that you want to be prepared to defend all points within your base. Focusing on one spot to defend is never a good idea since eventualy the opponent will find a way around it.

A good player knows when to defend and when to attack, there is a balance between the two.

Playing only offense leaves you greatly open to any attacks that may occur after you have just been killed off by someone who is defending and counter attacks. Playing strictly defense isnt good either. This leaves you trapped in a way because you will confine yourself to one area while the opponent gets a chance to expand and build up all over the map.