The difference between a bivy sack and bivy shelter

Marmot Alpinist bivy sack, 15 oz.

A bivy sack originated as protection for your sleeping bag, a "bivouac sack."  A bivy shelter is a hybrid between a solo tent and bivy sack.  The bivy sack was developed for climbers who wanted protection for their sleeping bag on multiple day ascents.  Adopted by lightweight and ultralight backpackers as a tent alternative, thereby keeping weight down, it continues to grow in popularity.  The bottom is generally urethane treated nylon to make it waterproof.  The top is made from a lighter nylon which has been treated with a laminate which is  waterproof or water resistant but breathable to some degree.  The Marmot Alpinist, pictured on the left, has a water resistant top, waterproof bottom. Except for a tarp, there is nothing lighter out there.  You say impossible?  Keep in mind the first documented through hiker, Earl Shaffer, in 1948 carried no tent.

Bibler Tripod bivy shelter, 2lb.10 oz.

The bivy shelter, pictured on the right, has expanded head and foot room also comes with bug netting.  Freestanding and easy to set up, you gain one pound ten ounces in the process.  Designed for a backpacker, not climber it helps avoid the claustrophobia felt by some when inside a bivy.   This model, the Bibler Tripod, has good venting capabilities to keep condensation at a minimum. 

Bivy sack sales still outnumber bivy shelters, probably due to the increased weight.  With a bivy and a sleeping bag liner you may, (depending on the weather) be able to eliminate carrying a sleeping bag in summer. 

REI lightweight fleece bag liner, 15 oz

With a sleeping bag liner, you not only keep the inside of your bag clean but also increase the low temperature rating of your sleeping bag by 5-10 degrees.  At fifteen ounces it may save you from going out and purchasing another sleeping bag just for cold weather.  This REI model is reasonably priced.  If warmer weather is in the forecast, a silk bag liner can give you a feeling of posh luxury out in the wilderness, where you would least expect it.  It also helps with the condensation problem inherent to bivies.

For reviews of other gearand essentials:

Water purification

Sleeping pads

Making your own gear

Meals

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