Pros:

Her damages are really high during all the game without needing any type of ofensife item.

Good ganks.

Easy jungle.

Good doing dragon and baron with the spider form and good securing buffs with the q.

Strong any time of the match.

Cons:
Skilshot cc.

Human form is mana dependent.

There are better champs to tank.

Hard escape out of the jungle.

She is good in ad top and mid compositions or with tanky ad toplaners like shyvana or renekton, because her AP damage cant be ignored but she is too week to be the only tank of the team. She is a good bruiser so its also advisable to have a hard cc support who can protect the adc alone.

She is also strong against high health champions *** volibear or nunu because of her health % damage and agains no gap closer champ because she can easily kite with the cacoon.

FARMING TIME
Is important to start the camps with the human form to dealt more damage with the q (it dealts x+8% of the actual health) and to change immediatly to the spider form to use the small spiders at much as possible, take into account that the spider form q deal x+ 8% of the missing health.
U can use the little spiders to tank but they dealt high damage and with the w u restore health when they attack so don´t let them die too soon. When u are moving from a camp to other remember that with the spider form u have 10 more of movement speed but with the human form u can recover the small spiders.

GANK
There are 2 different combos to gank with Elise.
1- If u start ganking in the human form u have to try to stun the enemy with the e and after using q and w to dealt damage became into the spider, remember that the q in the spider form can be used as a small gap closer if the enemy is quite far.
-Pros of this gank:

Uses all Elises damage.

U can use the rapel to catch the enemy if he flashes.

If u land the cacoon the enemy is dead.

-Cons of this gank:

Is hard to land the cacoon

Your enemy has 2 be paying 2 agresive to have the chance to catch him.

2-If u start the gank as spider use the rapel into a minion or an enemy and immediately transform into human to land the cacoon.
Pros of this gank:

Easier cacoon to land.

Useful line gank.

More succes at ganks.

Difficult to be cached with a counter gank.

Cons of this type of gank:
It doesnt use all Esises damage.
The cacoon isn´t a surprise so can be dogged if the enemy has good skills.

TIP: If u use the rappel at the tower it stop targeting u so it can be useful for early ganks.

Elise is a strong pick at late game, with good damage and quite tanky with good tools to kill the enemy ADC. She isn´t a good split pusher so she has to be always with the team. Her q smite combo and the rappel can be good tools to secure or steal Nashor or the dragon.
Before the teamfight starts u can poke with w at the human form, but never use the q because the range is 2 short and u can be catch out.
Once the teamfight starts if the enemy team is the one who initiates it try to stun the enemy bruiser who is more close to the ADC and when u see a chance to catch the enemy ADC use rapel to get her, but always try to save the rappel to jump into the ADC.
To initiate a fight you can try to catch someone with the cacoon or jump in with the rappel, but in that case make sure your team can follow you, because the rappel has not cc so it isn´t the best ability to iniciate and u can be insta deleted by the other team.

I hope u liked my guide, it isn´t done 4 expert people but i think it can be useful for all the people who want to start playing this op champ and don´t know even the basic things about her.

TNX 4 reading it and GL&HF on your next matches.

PD: Sorry for my English.