There is a wide variety of squads available to the eldar player, from the humble guardians to the deadly mimes (see my harlequin page). This page will discuss each type and advise on how they can be most effectively used. This is very important because squads are the core of any army.
These are the most important part of the eldar army. They make up the bulk of the eldar armed forces and they are very flexible. They can be used in a wide variety of roles. Some people see them as worthless cannon fodder and only use them to shield their aspect warriors, if they use them at all. This is a foolish tactic and players who do this are putting them selves at a great disadvantage.
So, what else can you do with guardians then, well to start with they have access to the shuriken catapult, this is a great weapon, especially against space marines with that -2 save modifier. This means that you can create very effective and mobile fire teams of about five or six guardians for well under 100 points.
The lasgun carrying guardians are also useful, the lasguns are quite effective against weaker opponents such as gretchin and imperial guard and the cheap cost allows you to field a larger army on the board. I also make sure that all my guardians have some sort of hand to hand weapon (even if it is only a laspistol) so that they are not completely helpless when it comes to hand to hand fighting.
Guardian squads can also be equipped as assault squads. However, although they are cheap they are no match for true close combat specialists. They can be used successfully, though, by using them to support characters or aspect warriors. One example is to mix them in with howling banshees against a well armoured foe such as a space marine terminator, where that guardians can take advantage of the termy being stunned by the banshee charge.
Guardians can also use jet bikes and man eldar support weapons and vehicles, these will be dealt with in the support section.
To sum up, I think that guardians are essential for just about any eldar force (except harlequins). As a rule of thumb, I would recommend having at least one guardian squad per 1000 points of army.
Dire Avengers
These are the eldar equivalent to the space marine tactical squad. They are common and very useful. They make great fire teams, I usually use four per squad. This makes the squad under 100 points, but still powerful enough to kill a lot of enemy troops. I tend to see these as improved guardians, and at just 22 points they are very cheap compared to most aspect warriors.
Dark Reapers
These are the eldar heavy weapon squads. They have one major disadvantage over the equivalent units in other armies. This is their limitation of only being able to use missile launchers. This means that they do not have the anti tank ability of lascannons or the troop killing ability of heavy bolters available to them.
They can take a range of missile types though, including my favourite, the plasma missile. They also have the range finder helmet which negates modifiers for the target's speed. This makes them the best anti light vehicle force in the game. They can even hurt some tanks and dreadnoughts as well. The plasma missiles are great against vehicle squads, because even if they don't damage the enemy directly the blast can often push the enemy vehicles into each other, causing collision damage.
Fire Dragons
These troops suffer from the fact that their melta guns only have a 12" range. This limits their uses considerably. I find them most useful when used as a body guard for the farseer as their guns are quite capable of killing would be assassins. They can also be used to attack enemy tanks because their guns have got very good armour penetration when used in close combat.
Warp Spiders
These guys are very mobile and have one of the most deadly weapons in the game. They are especially useful against orks and imperial guard. They are quite expensive in points though. I would not recommend taking large amounts of them because it is considered cheesy.
Swooping Hawks
Swooping hawk are the eldar jump pack troops. They have a much different role than other army's jump packers though. They are not very well equipped for hand to hand fighting and their lasguns hardly make them into great fire teams. Their best weapons are their grenade launchers and jump packs. They can make 36" long bombing runs across the board and drop grenades on the heads of their enemies on the way, when they land they can also fire their lasguns. They can also use their mobility so that they can use their krak grenades against vehicles in close combat, although the use of frag defender of electro-hull gives vehicles pretty good protection.
These troops are most useful at harassing and annoying the enemy as they can strike just about anywhere which instils a kind of paranoia and often he will have squads hanging around in his deployment zone trying to protect it from the threat from above.
Shining Spears
Jet bikes will be dealt with in the support section.
Howling Banshees
Howling banshees are some of the best hand to hand specialists in the entire warhammer 40k game. They are well armed with power swords and laspistols, they have a 6" movement rate and they have the banshee mask which makes their charge deadly. A charging banshee will beat most characters in close combat in the first turn and the strength 5 attack and -3 save modifier mean that they will often wound or kill their opponent at this stage.
This makes them ideal for counter attack situations when enemy troops have attacked the eldar front line. Banshees can charge down and destroy the spearheading enemy troops.
I think that banshees are great against any enemy, but they are probably most effective against space marines and make an ideal counter to assault squads.
Striking Scorpions
These are also a hand to hand specialist aspect. They are better protected than banshees with a 3+ save. They have less powerful close combat weapons but their mandiblaster helmets activate whenever they charge or are charged, this is often enough to kill weaker opponents before the combat has even started.
This makes striking scorpions very effective against numerous but weak troops such as imperial guard and gretchin.
Harlequins
Harlequins are very good close combat troops. They come equipped with a large variety of hand to hand weapons which allows them to be effective against a large range of opponents. They also have very good stats with weapon skill 5 and movement 6. Their main drawbacks are that they become expensive when fully equipped, they have no armour available except for fields and their minimum squad size is 10. This tends to mean that it is usually difficult to include harlys within smaller armies.
Mimes
Mimes are a special harlequin troop type that can infiltrate. They are also available in smaller squad sizes. They make a great spearhead to the eldar attack as they can usually be in close combat with the enemy from turn 1. See my harlequin page for mime squad stats.
Scouts
Scouts are basically guardians with needle sniper rifles and camelioline that can infiltrate. They are not the best snipers in the game but they are quite effective against troops with poor armour such as orks. The infiltration ability should be used to take advantage of cover outside of the deployment zone. Do not put them too close to the enemy because their guns have a 32" range and they are far from effective in close combat.