Beginning at the outside of the call, is the tape.  The tape is the part of the call that stops the air from moving around the reeds and focuses it between them and your tongue to produce sound.  It comes in many different styles and cuts, and ranges from soft pliable tape, similar to a Johnson and Johnson surgical tape, to a rigid tape, to an almost plastic textured tape.  Finding what's best for you is no science, it's purely what fits best and is most comfortable for you.  There is no right or wrong here.

The tape is folded over the frame of the call, and as mentioned above, is designed to create a seal in the mouth that will direct the flow of air, not around, but down the tongue and straight out through the reeds.  Any hissing sound is indicative of a bad seal, and means that air is escaping.

The tape is folded around the frame of the call and then cut around the outside of the frame. Most calls have more tape than necessary, so some can be trimmed off to make it fit right.
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Next is the frame.  Frames, back in the day were made of materials such as lead or tin because it was easy to mold, and the pliable nature of the material allowed it to be folded over the reeds.  As call making progressed, the unhealthy nature of walking around all day with a hunk of lead in your mouth was noted and the, less than lethal, aluminum frame was created.  There are some companies that utilize synthetic materials for frames, and they work fantastic, but the norm is still aluminum.

The frame is folded around the stretched reeds to hold the tension in place, and create the rigidity of the call.
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One other small piece of the frame that plays a big role is the tab.  The tab is located on the back of the frame and folds over to not only hold the frame together once stamped, but it also grabs the back of the reeds and clamps them down as well.  This is a fantastic way of maintaining the back tension of the reeds of the call so it won't loosen up.  The tab is also used to identify which side of the call goes up or down.  For most calls, the tab goes down when placing the call in your mouth.

Lastly are the reeds.  The reeds are located at the center of the call, and held in place by the frame.  Usually made of latex, prophylactic, or some similar form of thin rubber, the reeds are the part of the call that create the sound.  The sound that comes out of the call depends on the type of material, the thickness of the material, the tension or stretch put on the material, and any cuts made in the material.

The thinner the reed, usually, the easier the call is to blow, and softer more subtle sounds can be made at lower volumes.  Higher pitches can also, usually, be achieved much easier on thinner reeds.  But, with the ability to blow with less air, comes less volume.  So if you're in need of a mouth call that can cover some distance, a thicker reeded call may be for you.  Thicker reeded calls tend take more air to blow, and can produce greater volume.  This can come in handy in wide open areas or on windy days when sound doesn't travel as well.

Stacking reeds on top of each other, with different thicknesses, or different positions in the frame, can give the call the high to low note sound found in a real turkey yelp.
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The longest reed, or the top reed of the call is usually the one that gets the cuts made in it.  The cuts in the top reed will produce the raspy tones of a hens yelp.  This does not necessarily make it better, just different.  The cuts can also give an illusion to the user that it is easier to use, because the raspy sound can hide the imperfections in the users calling.  This can help a less experienced caller sound better at times, but can also deter them from getting better at calling by allowing bad calling habits to continue.  

When the tape, frame and reeds are put together, a musical instrument is created.  With the movement of air through the reeds, the sounds of the hen turkey can be made.  Whether you need volume, high pitch, low pitch, raspy, or clean sounds, there are tons of calls out there that can help you achieve the sound you're looking for.  Just find a call that is comfortable and sounds good to you.  Confidence in your calling is half the battle.  Good luck, hunt hard, and hunt safe.