Experience Mechanics

Killing enemies grants experience towards the weapon used for the kill, and all kills grant experience towards your warframe. Missions give bonus experience upon completion equal to ~50% of all normal exp gained through mission (appears to grant slightly above 50% due to rounding up).

Solo Experience
Experience gained by killing enemies is equally distributed between the weapon used to kill the enemy (including Sentinels if it killed the enemy, see section below for more info Sentinel Experience) and your Warframe. If the enemy was killed by an ability, your Warframe will gain all the experience.

Some examples to illustrate how this works:
 * Suppose a player kills an enemy that grants 300 experience with a Warframe ability. He will gain 300 Warframe experience.
 * Suppose a player kills an enemy that grants 200 experience with his Rifle. He will gain 100 Rifle experience and 100 Warframe experience.

Team Experience
Whenever any member of the team kills an enemy, that member's teammates will still gain full experience from that kill. This experience is divided equally among their Warframe and weapons.

Some examples to illustrate how this works: There is a distance in which a player must be to earn experience from a member of the team (range undetermined for the moment).
 * Suppose Player A kills an enemy with his Warframe ability, and the enemy was worth 500 experience. His Warframe will receive the full 500 experience, while his weapons receive none. His teammates will receive 500 experience each: 125 to their Warframe, 125 to their rifle, 125 to their pistol and 125 to their melee weapon.
 * Suppose Player A kills an enemy with his Pistol, and the enemy was worth 1000 experience. His Warframe and pistol will receive 500 experience each. His teammates, Players B, C and D, will also receive 1000 experience each: 250 to their Warframe, 250 to their rifle, 250 to their pistol and 250 to their melee weapon.

Team Bonus End Experience
Bonus experience is gained at the end of a mission completion. It is shown as an additional gold bar on your mission end screen. Quitting a mission revokes your bonus experience. All bonus experience listed below gets a bonus based on number of team mates, solo it appears to be 25%. You gain bonus experience based on mission type, number of additional team members and other attributes:
 * Kills: You get a bonus 50% experience from all kills at the end of mission.
 * Unspent energy: 1 exp for every 1 unspent energy at the end of the mission.
 * Lockdown: 50 exp - normal (25% bonus at end of mission -13 experience).
 * Hostage (Mission Type and Secondary objective): 100 exp - normal (25% bonus).
 * Raid (Mission Type): 200 exp - normal (25% bonus).
 * Number of Team Members: Requires investigation.
 * Perfect Run: No deaths? No Detection? Requires investigation.
 * Affinity Orbs: Affinity orbs grant you 100 experience to your warframe. It isn't subject to bonus.
 * Affinity Booster: This is a store bought booster than doubles ALL affinity gains.
 * Mastery Rank Test: 50 exp or 100 exp (depends on the test).

Extermination, Assassination, Spy &​ Sabotage mission types give no bonus experience for completion.

(Still need to check the rest of mission types. There is no penalty (yet) for deaths & revivals in missions. Must check bonus xp for team mate revivals.)

Note: Sometimes the mission end screen does not correctly display your normal and bonus experience, but your total experience is still the same. For example, it may show that you earned 0 normal and 12,500 bonus experience when it was supposed to be 10,000 normal and 2,500 bonus experience.

Sentinel Experience
Experience gained by Sentinels functions in a similar way to Warframe and weapons. Sentinels will gain experience from the following sources:
 * Precept kills (50% to Sentinel, 50% to your Warframe)
 * Precept uses (100% to Sentinel, sometimes, but not always, indicated by a XXX Power pop-up when a precept is used)
 * Your weapon kills (~22% to Sentinel[1])
 * Ally kills (X% to Sentinel?[1])

[1] this experience is not taken away from what your Warframe and weapons would normally gain from kills, but is simply 'extra' experience.

Sentinels will not gain experience from the following sources:
 * Sentinel weapon kills (X% to Sentinel weapon, X% to your Warframe)
 * Your Warframe ability kills (100% to your Warframe)

(needs further testing to determine what percentage of the experience the Sentinel gets)

It seems that Sentinels will not keep any experience gained during a mission if they die and are not revived (whether by Regen or by reviving with a player) by extraction. (further testing is required)

Enemy Experience
Killing an enemy nets you experience, which is shown on screen as a number underlined in blue. To determine the scaling of enemy experience values, use this formula:

Total Experience = (current_level) ^ 0.5 * 0.1 * base_exp + base_exp
 * Base Exp: this is the base experience value for the enemy.
 * Current Level: somewhat self-explanatory, this is the level of your target enemy.