Runes are all armor pen and magic pen (so more of your damage actually hits them). The remainder are CD reduction to make you able to spam your abilities.

The guide here shows low att dmg and low ap, but in the game, the 2% of your manna you gain from manumane will give you an extra 80 dmg. IN addition to this, the rod of ages you get will increase both your att and AP when it gains AP, health, and manna gradually. Similarly, Trinity force gives AP, Att, manna, and health (the manna will increase your attack even more). I usually get a black cleaver to get even more armor pen since your q has on hit effects. Depending on the situation, I may switch it out for a madred's razor (for the on hit effect) or a last whisperer (for super tanky teams). You can also get a youmuu's ghostblade if you like having a ghost substitute. Personally i might switch out manamune at the end of a game for madred's bloodrazor so that i get the extra on hit effect. usually by that time in the game, every character is kinda tanky in hp so its always good to have. If you do not like spamming abilities as much, you can also replace yourboots with another trinity force.

i begin with a mana pendant and 2 hp pots
next i buy a vampire secpter so i dont have to waste as much cash on hp pots
i get boots or finish my manamune (depending on how good at last hitting i am or how good i am playing)
i get sheen
i rush a rod of ages after this (usually starting witht he hp and manna crystals so i am not as squishy)
i upgrade my boots for the CD reduction and movement speed (although 375 is still not that blindingly fast)
after the rod of ages i get the black cleaver so i can wreck ppl
this is followed by the hexblade so that i heal from both ap and ad attacks
i finish with a trinity force (if you are wondering why i dont rush this item sooner is because i find that you can easily come up with the cash for this item more easily late game whereas the other items in this build are not as expensive)

Pros
high damage output
fun to play
gets cheap kills with ult.
good at tower defense (if people dive you, they had better stun you otherwise you can just arcane shift out of danger while they get raped by your turret)
late game ability spamming
Cons
no stun
slow at the beginning
squishy at the beginning
all skill shot abilities make it hard to hit players using flash or faster characters (like master yi)

Generally you have to be good at last hitting. However there are a couple of games where I could not harass the other player because he would just heal (taric or allistar who had clarity on them). when this happens i generally give up on harrasing them and just try to out farm them. You can use your q to last hit minoins very effectively.

As a rule, i usually try to get blue buff early game and red buff mid game. By end game I usually have both by stealing them from the enemy. Blue is strong on ez because it gives cd reduction and manna which he uses up very quickly. Red is also strong on ez because he has its effects applied on his q.

I personally like to get all 3 abilities as fast as I can. Then I follow up by upgrading both of my skill shots. Since this build is favoring AD ezreal, i usually upgrade my q. I generally only use r e to set up my skill shots or to jump walls to get out of a sticky situation.

Flash-with arcane shift you can make getaways (or chase ppl but i generally just try and line up my ult correctly)
Ignite-this is personal preference, i like the additional 10 ap it gives after i use it and it is great early game for getting kills (i also use it to get blue buff quicker when im a low lvl)

other viable spells
ghost-great getaway spell or for tracking down fast enemies (must rearrange masteries)
exhaust-so you can stand a chance against dps with lifesteal and high dmg output (tryndamere and master yi-no this build cannot one v one a well built one of those characters)
cleanse-if you are prone to getting stunned (generally if you are that close to the enemy that they can stun you before you can deal damage to them you are way too close... you're attacks are long range so take advantage of it)
clairvoyance-if you want to be a sniper with your ult.
teleport-good for any character

If you're at mid, use arcane shift to line up your essence flux and mystic shot. That way if you land all your shots you do massive damage and your cd will be reduced by mystic shot. I also throw in ignite into the mix for early game when it is still effective. I'll use arcane shift, ignite and fire my two skill shots. This way you benefit from the extra ap from your masteries to maximize your damage output.

The great thing about ezreal is that you dont even have to be involved in a fight to help. since your ult is global you can backdoor and shoot it to help your teammates. however, i like to be involved in the fight. you should always be behind your allies. This is not to say that you cant come forward to harass with your skill shots. but have arcane shift ready to get you out. also be aware if any of the opposing champs have a cc that could potentially end you.

All in all this build really depends on you landing your skill shots at the beginning. while when late game comes, it doesnt really matter as much because of your HUGE manna pool and CD reduction.

TIPS for survival
-try to get mid because i usually begin the game with a mana pendant and 2 hp pots making me very squishy (but in a 1 v 1 its hard to kill me because of my dmg output) however, be wary of healing mid players such as vladamir, taric, and nidilee
-pick off cheap kills with your ult from mid (i usually will run back behind my tower so that the othr mid player cannot see me, that way there is no way to warn other players on the field or to dodge it himself)
-learn to use arcane shift (there is no other way to put it u must learn how to use this both offensively and defensively-if you can land this to hit the enemy champ as well as your othr two abilities it'll usually take a huge chunk out of their health)
-survive (if you can last the first couple of levels with ez b4 blue pilling or getting killed you should be able to own... if not it will be a long time till you can start owning players)
-get sheen quickly (although i dont usually get trinity force until mid to late game, i always get sheen early... its effect is a must with your one two hit of essence flux followed by mystic shot-an applied on hit effect of 150% of the dmg)