Hey guys, first build I've done so constructive criticism is very much welcome.

I play Ezreal as an AD (not a fan of AP as it is quite lacking since his nerf). I focus on dealing as much damage as possible, which explains the overall squishy-ness, but we will get into more depth about that later. I prioritize on staying at the back of team fights, as ranged AD carries should, and dealing as much AoE and single target burst as possible, while supporting allies by granting more attack speed. With great positioning and practice, you will be able to melt the faces right of the enemy team and get epic snipes, map not needed of course. But I digress, let's continue with this build.

Runes for Ezreal can be played around with to suit your need. After much practise and trial and error, I have found these runes most effective (see top of page). These runes allow you to deal the most amount of damage you can in the quickest span of time, let me elaborate further.
Marks and Quints: Armour penetration to maximize damage early game so you can get fed right off the bat. They also help mid to late game in combination with Black Cleaver to really shred through the enemies armour.
Seals: Attack speed is your best bet for two reasons. First, it makes your passive a lot more useful early game when your attack speed isn't too high. The runes synergize great with your passive and should allow you to out auto-attack anybody (except Fiora cause she's a... ANYWAY!)
Glyphs: I take cool down glyphs because, once again, it helps in many ways. It goes great to charge up your passive at any point in the game and lets you reach max stacks faster. Also, the CDR helps greatly with your E ability ( Arcane Shift) and your ult, allowing you to spam both and deal tons of damage while staying safe.

As you can see from the mastery tree above, I take everything to max damage. The part where this guide may differ from others is the 9 leftover points. I put them into the defensive tree because Ezreal really needs that little bit of extra tanky early game to allow him to get fed and stay in lane longer. This is all situational however. If you feel you would be better off getting the extra mana regen as you like to harass a bunch, go ahead. However I do not believe this to be a smart choice. The mana you get from sheen should be enough to keep you happy and allow you to harass. In addition, you should also be harassing with your auto attacks, not just your Mystic Shot as this will deplete mana quickly. This is a mistake I see often, auto attack and you will be okay.

Ionian Boots of Lucidity, Berserker's Greaves, and Mercury's Treads
These are all viable options for boots. I prefer Ionian Boots of Lucidity as it helps me charge up my passive and spam my ult a little bit more. They also go great with the CDR runes I have.
Sheen Trinity Force
These are Ezreal's core items. They will boost your damage output STUPID amounts. Since your Mystic Shot applies on hit effects, it will proc, as will as deal the Trinity Force damage, allowing your burst to be that much better. Once you have your Sheen, your harass in lane will be unmatchable.
The Brutalizer can be bought after Sheen or Phage. This item is meant to get your DPS and CDR up for cheap. If you are having trouble in lane, get this item early as it is sure to boost your burst and overall DPS
Black Cleaver
This is another important item. It will give you even more attack speed to shred through your enemies. It will dramatically increase DPS and help against those tankier enemies you may be going against.
Infinity Edge Bloodthirster
These are just late game items you may want to get if you are farmed up and want even more damage. The Bloodthirster will give you some nice life steal and AD which is very helpful.

When it comes to summoner spells, I believe Ezreal is one of the most flexible champs in the league. When I first started playing Ezreal, I took Clairvoyance for my ult and Flash. However, I have grown since then and this is what I recommend.
Flash Ghost Ignite Heal Exhaust
A combination of any of these 5 spells are surely your best bet. Ezreal has a built in Flash, but a second one is still really nice (similar to Ahri, has 3 flashes but a fourth never hurts).
Smite Uhhh... hmmm... I mean... you could give it a shot!

Thanks for reading through my guide guys. As I said before, this is my first guide, and I'm not all too good at doing this. I hope people will leave some comments on how to improve so I can pump out some greater guides in the future!