Dana (Sanskrit for giving) is the practice of the lay members of the Sangha.  The Buddha taught that the responsibility of the monks and nuns was to engage in self-cultivation and to share the fruits of this cultivation with the lay members of the Sangha.  They were admonished to teach the Dharma to any who asked.  The responsibility of householders was to support the monks and nuns in this endeavor through the offering of food, cloth (monks' garments), and medicine.  Later, shelter was added to the list of things that could be offered to the Sangha.

In practice, this is seen when donations are made at temple for the upkeep and progress of the temple. The donations that are placed in a donation box, or boxes, are put there for that reason.  Food is also brought to the temple for the feeding of the monks and nuns and any lay people that may be in attendance.  Personal money for the monks' use is handed to them in an envelope.  This money is a necessity, used to purchase such things as material (for robes or the shirt-and-pant suits they wear under their robes), eye glasses, copies of sutras, medicine, toiletries, sandals, socks, etc.  Monks who travel teaching the Dharma may also have to pay car insurance and maintenance.

To complicate matters, from the standpoint of the traditional rules guiding the actions of the Sangha, there are a few pioneering monks who work with Americans and have found it necessary to work a job in order to found and maintain temples.  Monks who work at jobs to support temples and the formation of American sanghas are present in the Japanese, Korean, Chinese and Vietnamese traditions. 

Those envelopes are called  lì xì  in Vietnamese* and they are the traditional way to give a gift of money, especially during the New Year.  They're used any time discretion for a monetary gift is in order, and are akin to the American greeting cards with a slot for cash or checks (without advice or sentimental poems). 

The amount given by way of lì xì isn't fixed, it's the thought and act of giving that are important.  However, in Asia the amount would traditionally be an even number, as odd numbered amounts of cash are given at funerals.   In the United States, the odd/even symbolism isn't followed -- often a single coin or bill is enclosed: quarters or one-/five-dollar bills are perfectly acceptable. 

In China, the color red is considered is good luck, and may have developed from a folk custom during the Qing Dynasty, when coins tied together with red string (yāsuì qián) were believed to protect the elderly from sickness and death, each hundred coins given tied in this way would double the elder's lifespan. 

Today, li xì are traditionally given to youngsters by adults or to unmarried people by those who are married.  Note that any child or unmarried individual is eligible, but it would be shameless for an adult - 21 years or older - to request a red envelope.  Because of the Buddhist tradition that monks do not handle money, this type of gift to a temple or monk is often folded in paper or an envelope. even though a cashless life is very difficult & rare today. 

*Names for Red Envelopes

    China - lai si (Cantonese, also transcribed lai see),  hóng bāo (traditional Chinese - pinyin), and during the Chinese New Year yāsuì qián (traditional Chinese).

    Vietnam - lì xì (similar to the Cantonese pronunciation, laisee).

    Note: in Japan, otoshidama (a monetary gift) is given to children by relatives during the New Year in a white envelope with the name of the receiver written on the back.

• Sun nin fai lok (Cantonese for Happy New Year)  
• Gung hei fat choi or Kung hei fat choi (Cantonese for Congratulations and be prosperous) 
• Suìsuì píng'ān (everlasting peace year after year), said out loud to no one in particular if an object has been broken during New Year to avert bad luck (based on a pun)
• Although not common, even in the United States, children may amuse themselves (if not their elders) by saying Gōngxǐ fācái, hóngbāo nálái (roughly translated as "Happy New Year, now give me a red envelope.")  Use at your own risk!

Happy New Year!