In warhammer each creature's abilities and skills are measured by a standard set of statistics. These values are:
The number of inches the unit can move in one battle turn. This will determine how well you can manoeuvre and respond to enemy surprises. It also has a bearing on how fast your army banners move on the map. Units with high M values (usually cavalry) are very useful, and well worth purchasing. Some units can fly, in which case they can move 24 inches each turn! Flying troops are quite rare. The warhammer base level from which all others are compared for M is 4.
(WS) Weapon Skill
The skill of a trooper in close combat. The higher the weapon skill, the better in warhammer, but more important is how high your skill is compared to the enemy. If its higher, great! If its more than twice your enemies even better. If its really low, dont worry too much, your men are probably cheap anyway. The warhammer base WS is 3.
(BS) Ballistic Skill
The skill of a trooper at range. Again the higher your BS, the better shots your men are. However, BS doesnt improve the power of a weapon - so pick your ranged weapons carefully. Here is a quick guide to the strengths (see next stat) of a few weapons:
Bows (all kinds) 3, Crossbow 4, Handgun 4/5, Javelin - same as thrower.
The warhammer base BS is 3.
The power of a troopers physical blow. The strength of your troops is very important as not only does it allow you to wound your enemy, but it reduces the effectiveness of their armour too. It is important to note with strength that to wound the enemy you must compare it with the enemies toughness (see next stat) - so radically high strength (6 and above) make mincemeat of average (toughness 3) troops. On the other hand super tough troops (like cannon chassis) cannot be hurt at all by the likes of normal troopers (strength 3) or bows.
The warhammer base S is 3.
The physical resistance a trooper has to harm. This was really sort of covered above. Note champions of most races have the same toughness as the units troops within which they fight - only heroes and generals have higher normally. The warhammer base T is, surprise, surprise 3.
The amount of damage each trooper in the unit can take before dying. It is rare for troops to have more than 1 wound, but very useful. The base W is 1.
The speed of a troopers reactions in combat and other situations. This determines who fights first in combat after the first turn (the charging side always goes first - see fighting a battle below) - so in some cases your men might not get to fight as they are killed before they even get a go! The base warhammer I is 3
The number of times each turn a trooper can attack in combat. This stat is very important in combination with other good stats. If you have 10 attacks, WS 10, but only S 1 you arent much of a threat, and vice versa, if you have S10, WS1 and A1, you might be very strong but you cant attack very often - and rarely hit when you do. The warhammer base is 1, so more than is highly sought after.
The charisma, willpower, cool and intelligence of a trooper. This is a truly battle changing stat. Your heroes and generals can improve your units Ld by being in or near (respectively) them - so dont forget this in your army lists and plans. Ld is tested against whenever your men might run away basically - when shot at, beaten in combat, scared or panicked. The base stat level in warhammer is 7, and seeing as all tests are taken on 2, 6 six sided dice, this mean a troop with Ld 7 has 7 in 12 chance of not running away - not much over 50/50, but enough. Troops with Ld under 7 are truly unreliable, and desperately need good leaders. Those with Ld 8, 9 or even 10 are very good investments.
Now you have an idea which stats are more important than others when looking at the troops you have available - and which may become available in the future to you. No stat can go above 10 - and other than Ld, it is rare to see them above 4 or 5 in most cases, so be on the lookout for things with many wounds, high strength or lots of attacks...