Jax is a character you really shouldn't mess with. He Is highly dangerous in team fights and can be unstoppable if fed.

Warning: If You Have Poor Reflexes or TUNNEL VISION Do Not Use This Character

Basically, If You Don't Get Kills or allot of Creep Kills Early Game You Wont Snowball As Easy And Become Unstoppable. Simple Isn't It

Pros: Able To get Away Easily.
High Burst Damage Mid Game
Easy To Play

Cons: Low Damage Early Game
Unreliable Stun
Unreliable CC (Crowd Control)

You Must get Allot Of Cs (Creep Score)(Creep Kills) Or Get a lot of Kills. If You Have a lot Of Creep Kills You Will Most Likely Snowball Quicker. (Very Obvious)

If Your Friend / Lane Partner Is Getting Chased Try To Use Your Stun To Get Them Off Him/Her. But If You Don't Have Stun Ready You Cant Do Anything To Stop him But Try To Attack Him. Go Into Team Fights First If Your Team Doesn't Have Tank. Remember, Your Really Hard To Kill If You Play Right.

Jax's Jump Allow Him To Jump Over To Object (Can Be Used As A VERY reliable escape ability)
Jax Has an unreliable yet, valuable Stun.
Jax's Empower allows him to Deal More Damage The Next Hit Usages : When about to Jump / or If jump is on CD (Cool Down) When About To Hit With Your Ult's Passive.
Jax's Ult Active- Gives Him Magic Resist And A Hgher Dodge Rate
Passive - Every 3rd Hit Jax Deals More Damage. ( I Know It Sound Like A ****ty Ult, But It Can Save You At Times)