What determines how much experience a champion receives?

How is experience gain determined when killing minions and/or other champions? Does last hitting change experience? Distance from minions? Is experience for champion kills handled the same way?

From the LoL Forums

For minions on Summoner's Rift:

• Melee minions are 64 exp +5 every 3 minutes
• Ranged minions are 32 exp +3 every 3 minutes
• Rider minions are 100 exp +7 every 3 minutes

Time starts from game start, not from the initial minion spawn. Minions increase in exp if they spawn after the "x" minute mark.

Last hitting doesn't change the experience gained. Neither does distance - you either get the experience or you're too far away. That's why zoning is so effective.

From LoL Wiki:

Killing an enemy champion rewards a player with an amount of experience determined by the level of the champion slain. The experience gained is equal to 75% of the total experience required for a champion to go from the slain champion’s current level to the next level up. In other words, if a level 7 champion is slain, the total experience gained is 75% of the total experience required for a champion to go from level 7 to level 8, or 862.5 experience points. The experience reward will be increased if there is a positive level difference between champions (e.g. a level 5 champion killing a level 7 champion) and decreased if the level difference is negative.

EDIT: Additionally, you have to factor in +exp masteries and runes, if you have either of them or both.

In addition, when more than 1 champion is sharing experience, 10% extra experience is split among all champions.

• Two champions each gain 55%.
• Three champions each gain 36.7%.
• Four champions each gain 27.5%.
• Five champions each gain 22%.

To add to the above poster's information: don't forget that Zilean's passive, Heightened Learning, will also increase your team's experience gain by 8%.