Players can earn Glimmer in many ways, including defeating Majors and Ultras in all game modes, for each completed mission and PvP win, dismantling weapons and equipment, and for exchanging items found in the field with vendors.[2][3][4] Glimmer can also be found in various chests hidden across the map in caves and rooms.

Ref: http://destiny.wikia.com/wiki/Glimmer

Vanguard Marks are the currency used to purchase Vanguard weapons and armor from Vanguard vendors.[1] A Guardian can earn Vanguard Marks beginning at level 18, but they are limited to earning a maximum of 100 per week and an inventory cap at 200. Vanguard Marks are rewarded for completing Strikes in the Strike playlist, completing Public Events, and exchanging materials with Roni 55-30.

Ref: http://destiny.wikia.com/wiki/Vanguard_Mark#Vanguard_Marks

Crucible Marks are an exchange currency that can be used to purchase items from from Arcite 99-40 and Lord Shaxx.[5][6] Once a Guardian reaches level 18, he or she will earn 3 Crucible Marks for each win and 2 Crucible Marks for each loss in the Crucible up to a maximum of 100 Crucible Marks in a week.[7] Crucible Marks can also be earned by trading in 50 materials to Arcite 99-40.

Ref: http://destiny.wikia.com/wiki/Crucible_Mark#Crucible_Marks

Motes of Light may be acquired by various means. Every Guardian receives a Mote of Light from Kadi 55-30 once they reach level 20. Once a Guardian reaches level 20, any experience earned goes to acquiring a Mote of Light. They can also be awarded randomly for completing Crucible matches, completing Vanguard Strikes, in loot chests, or in any type of engram.

Ref: http://destiny.wikia.com/wiki/Mote_of_Light

Strange Coins can be obtained through numerous means. Completing a weekly Heroic Strike mission will grant the player three (level 22), six (Level 26), or nine (Level 28) Strange Coins. Completing a weekly Heroic Strike mission at a higher level after already completing it at a lower level the same week will award the difference in coins. So the maximum number of Strange Coins that can be won per week from this method is always nine. Strange Coins can also be randomly awarded for completing Public Events with a Gold Rating, completing Crucible matches, completing Weekly Nightfall Strikes, completing raid phases, decrypting decoherent engrams, or opening loot chests.

Ref: http://destiny.wikia.com/wiki/Strange_Coin

As for purchasing power, I would say that Strange Coins are the one to aim for, as this will allow you to buy exotic items from Xûr when he is in the Tower. None of the other currencies allow this. The Vanguard and Crucible marks will allow purchase of legendaries, but they take time to earn.

The Taken King simplified the Destiny economy by removing some currencies, adding others, and changing how they are earned.

Glimmer

Earned by:

• Finishing quests and missions
• Dismantling weapons and armor
• Some ghosts have a perk that gives glimmer when killing a certain enemy type
• You can find or purchase a consumable that gives glimmer when killing a certain enemy type for 10 minutes when used (such as Ether Seeds for Fallen)
• Enemies can drop an item that gives 200 glimmer when turned in to the Cryptarch (such as House Banners or Silken Codex)
• Some House of Wolves missions give an item that gives you 2,500 or 5,000 glimmer when turned in to the Cryptarch the first time you complete them per character

Glimmer is used throughout the game, generally in addition to another currency. Infusing a legendary item costs glimmer along with legendary marks and weapon parts, for example.

You can hold a maximum of 25,000 glimmer at one time. These are shared across all characters on your account.

Legendary Marks

Earned by:

• Finishing the Daily Quest and Daily PvP Playlist (15 marks, once per account per day)
• Completing quests in the Vanguard Heroic playlist (10 marks, 3 times per account per week)
• Winning games in the Featured PvP Playlist (10 marks, 3 wins per account per week)
• Some quests give Legendary Marks as a reward the first time you complete them per character
• Dismantling legendary weapons and armor (3-5 marks, depending on how much XP has been applied to the item)
• Completing the weekly Iron Banner bounties (25 marks)
• Completing some of the Trials of Osiris bounties (25 marks)

Legendary Marks are used to buy weapons and armor from the various vendors in the tower (including Lord Saladin when he is in the tower for the Iron Banner). They are also used to infuse legendary weapons and armor.

You can hold up to 200 Legendary Marks at a time. These are shared across all characters on your account. There is no limit to how many you can earn in a week.

Strange Coins

Earned by:

• Doing most anything in the game - quest/mission rewards, turning in bounties, dismantling engrams, faction packages, etc.

Strange Coins are used to buy items from Xûr, who appears in the Tower or the Reef from Friday reset until the Sunday reset. He sells 1 Exotic armor for each class, along with an Exotic weapon or armor engram among other things.

Silver

Silver can only be obtained by spending real money in the store of the console you play on. You get approximately 100 silver for $1, but can only be bought in certain bundles (such as 500 silver for $4.99). Buying more silver gives you additional bonus silver as well.

Silver is only used to buy items from the Eververse Trading Company in the Tower. Currently only emotes are available here.