Varus is a Ranged AD carry which I personally consider the best ingame. Varus works great at bot but he also can go mid if needed. Do NOT build AP Varus, he is not viable and there is a high chance you will fail.
This guide will cover the basic build, skill order, Pros & Cons, Creeping, Runes and Masteries.

Pros:
- Good Farmer
- High Attack Damage
- Stun/Slow and increased range
-
Cons:
- Not much survivability
- Very Fragile champion
- Need to be with a good support to get a nice and fresh penta. (or luck) :)

Varus build is just like any ADC but it needs to be followed strictly in this order.
1st: Get a Dorans blade, Some people might say that dorans is not for pros, bla bla bla, but guess what? It works for varus :D so get 1 of this just at the beggining.

2nd: Get boots of mobility and upgrade as soon as possible to Zerker Greaves, this will not only give you mobility but it will also make you attack really quick and when you get crits you destroy enemies.

3rd: Vampiric Sceptre, Some people think its a good idea to get a B.F sword first, its NOT!
Stay farming and 3 mins after getting the boots go back for this item and if its necessary get Vamp Sceptre AND B.F sword but the first item is Vampiric. ALWAYS.

4th: Infinity Edge, do NOT get Blood thirstier first, Get an infinity edge and save the Vamp sceptre for later while you get all the money to get IE.

5th: Phantom Dancer: This is a great item specially for Varus, It gives a ton of crit chance, attack speed and you guessed it again, our old friend MOBILITY. Try to get Zeal first rather than the cloak.

6th: Blood Thirstier. Yes, the Blood thirstier is now avaliable! Buy it as soon as you finish building the PD. Why? Because each time you attack (65% crit chance) + lifesteal = Instaheal in less than 10 seconds. Also, if there is a character dealing high damage to you, EXHAUST HIM!!!!! and attack like an insane beast, just run if its really necesarry and remember, the ultimate is your friend!

7th: The last slot you'd like to get is some penetration, a really nice item i'd recommend would be the Last Whisper, It gives a huge advantage and it has a low cost. Farm fast and get it.

8th: Sell the dorans blade, at this stage you can chose in getting some armor or deal even MORE damage!!! :D
For Damage i recommend getting Madred BloodRazor's. This combined with lifesteal and critical is complete ownage!

For armor i'd recommend, Thornmail OR Force of nature depending if their AD or AP carry is fed.

At low levels, Varus succeeds at farming, while on bot, just last hit like normally. The average creep score you must have goes like this:
-Minute 5 ~ 50
-Minute 10 ~ 100
-Minute 20 ~ 190/230
-Minute 30 ~ 300/350.
When varus reaches level 9 he can start attacking minions without last hit because he is excellent to farm and will always or nearly always get the gold.

~Always try to hit your Q not where the enemy is but where he is moving towards. If the enemy stays still throw the Q directly to him.

~The arrow storm is extremely useful, not only for damaging/slowing the enemy with the blighted quiver but also to run. If there are more than 3 champions against you solo and you feel you can get the 3 go for it, but if you dont feel like it, throw your ultimate when they are together, exhaust the ADC and throw a hail of storms in the ground a little after you are heading so they get slowed. Also remember you can trick them by firing the hail towards a place you didnt ran to.

~Make the best use of Blighted quiver. Max it last but always Q or E when the enemy has 3 stacks!!!

~The timing doesnt matter if you are on a bad situation just charge your Q for 0.5 seconds or 1 second and that will be enough to get the enemy in low health if he had some stacks of blight.