Affinity

Affinity is basically Warframe's mechanic for experience points.

Killing with a Warframe's ability will grant 100% affinity towards the Warframe
Killing with a Weapon will grant 50% affinity to the weapon, and 50% to the Warframe
Being near an ally when they kill an enemy, 25% affinity is granted to the Warframe, and 75% affinity is spread across equipped weapons, 25% split between 3 weapons equipped, 37.5% split between 2 weapons equipped, 75% if only one weapon is equipped.

There are multiple ways for gaining affinity to level your gear,

Grineer give the most affinity out of every faction, so it is recommended to run high level Grineer defense.
Helene, Saturn - This place is great for affinity as well as passively farming Orokin Cells, because you need a lot to craft prime gear.
Hydron, Sedna - Also a great area to farm affinity.
Elite Sanctuary Onslaught - The best place to level up weapons, because it is a onslaught of high level enemies to grant tons of affinity.
Adaro, Sedna - This place is a tricky one, It requires you to be stealthy, so it is recommended to bring someone with a Equinox and sleep everyone, because stealth kills stack up to 500% bonus affinity per kill, and like stated earlier, killing grants 50% to weapon and 50% to warframe.

Last updated 2 years ago