Energy Efficiency means different things to different climates.  A replacement window that keeps a house warm in Alaska might not keep a house cool in Texas.  Adams Home Windows is based in Dallas Texas, so lets tackle Hot Climates first.

Three things make your windows inefficient:

1. Radiation
2. Conduction
3. Air Leakage (infiltration)
   
   

Radiation:

Lets first get an idea of where heat comes from.  All electromagnetic waves contain energy (heat).  Shorter "ultraviolet" waves contain a higher amount of energy (heat) than longer "infrared" waves.  Of course, if we wanted to build the most energy efficient window we would build one that blocks all electromagnetic waves.  We call this a wall (attempted humor).  But if we want to be able to see through our window, of course it is necessary to allow visible light to pass through.  The solution is to add a metallic mixture called "Low E", based of silver or aluminum to the glass.  A measured amount of Low E successfully blocks a high percentage of the hottest UV wavelengths and still allows a high percentage of cooler visible light to pass. The amount that the Low-E blocks UV is labeled on each replacement window as its "U Rating".  If a replacement window has a U-Rating of .30, it has been measured to block 70% of UV (heat).  If it has a rating of .20, it blocks 80%.  Obviously, lower U-Ratings are better for blocking heat but it is very difficult to block more that 75% without also blocking some of the visible spectrum (tinted, dark windows).

Conduction:
If heat is applied to one side, how fast does the other side get hot?  This is "rate of conduction".

1. 1. Conduction of the Frame:
   Aluminum and Steel window frames are by far the best conductors of heat.  This means that if the outside frame heats to 100 degrees, the inside will quickly conduct to 100 degrees.  The hot frame heats up the cooler air (inside your house) and causes it to rise (hot air rises).  The rising hot air causes a vacuum affect that pulls cold air to the hot window frame.  Even if the window were perfectly sealed from the inside and out, this process would give the illusion that a breeze is blowing in.
2. Not only is Vinyl far less conductive but Energy Efficient Vinyl Replacement Windows are built with large air pockets incorporated into the frame.  This allows the inside of the window to remain room-temperature while the outside vinyl never heats to nearly the outside temperature.  Wood is by far the most energy efficient window frame material.  Billions of tiny air pockets within wood make it a terrible conductor of heat or sound.
3. 2. Conduction of Glass:
   There are other products that are being tested to one-day replace glass in windows but we are far away from that becoming reality.  For now, Glass is king and the king is highly conductive.  All Energy Efficient Replacement Windows are built with at least two panes of glass and in direct summer sunlight, the outside pane of glass is GOING TO GET HOT.  There is nothing that we can currently do about that.  The question becomes, "how can we keep that heat from transferring through the outside pane of glass, through the airspace between the two panes and through the inside pane of glass?"  The answer is Argon Gas (or any dense gas).  Filling the airspace between the two panes of glass with Argon Gas insulates the unit and reduces the speed that hot air rises and cold air falls.  This greatly increases the amount of time that it takes for heat to transfer through to the inside piece of glass.  It is important that the material that is used to hold the two pieces of glass apart (the "Spacer") is made of a material that isn't conductive.
   
   

Air Leakage (infiltration):
In the world of "Energy Efficient Replacement Windows", this is often the most overlooked factor, yet it's the one that is easiest to understand.  Any window that opens relies on seals and weatherstripping.  It doesn't matter how efficient the glass is or what the frame is made out of if hot air can infiltrate the window around the sides and under the bottom.  Picture windows are the most energy efficient because they don't open at all.  There is no weatherstripping.  Casement Windows open and close like a refrigerator door and can be sealed more tightly than other styles.

Be careful!  It is possible for a terribly inefficient window to achieve great efficiency ratings because manufacturers are usually only required to measure the efficiency of the glass.  If you can move the window sash around even after you close and lock the window, air infiltration could be completely negating the benefits of efficient glass.