How did WW1 start?

There are 4 MAIN causes

Military - Governments would build up their military not only to be safe, but also to be competetive with other countries. This was aggressive and became counter-productive.
Alliance - Alliances were formed such as: The Triple Entente, The Triple Alliance & The Slavic Allies of Russia. This seems like it would have prevented war, however this made countries more confident to go into war. This also created an effect like mountain climbers on the same rope: One goes in and brings the others with
Imperialism - Like militarism, the countries start by pursuing safety and became competitive. This happened with overseas land instead of military
Nationalism - Nations were somewhat "egotistical" making them more confident to go into war.

Fighting in WW1

I believe that the nature of fighting in WW1 was more aggressive than in WW2. There was a lot of appeasement in WW2.

Some of the weapons and vehicles used (according to history net) :

Rifles
Machine guns
Flamethrowers
Mortars
Artillery
Poisonous gas
Tanks
aircraft

To conclude

Many factors contributed to the start of WW1. The weapons that were used were devastating to Europe.
Germany wasn't dealt with correctly and was punished harshly and this led to WW2. This was called the treaty of Versailles.