First they are all concrete, just different application methods.
Gunite (dry nozzle)is a dry mix where water is added at the nozzle.
Shotcrete (wet nozzle) is premixed wet.
They both use high air pressure to force the mixture to stick to the wall. Just as if you take a ball of mud and throw it at something, if wants to stick.
Hope this helps.

Thanks a lot. I have always wondered this.

Interesting - I was looking at a PB's site & stumbled across this info. Truth or hype I wonder?:
(from http://www.houstonpools.com/shotcrete.htm)
SHOTCRETE VS. GUNITE - WHAT IS BETTER?

We will be updating detailed information as to why Shotcrete is far superior than the normal "Gunite" pool. Shotcrete is not only used for swimming pools but used for high performance applications such as DOT roads and expansion bridges.

THE TRUTH ABOUT GUNITE

Gunite is shot through the hose dry and the ratio of cement to sand mixture is at the mercy of your gunite company. All one has to do is walk the ladder and open the small hatch that covers the cement before they start shooting a new truck. Remember if you try to do that the Gunite company will not be very happy about it, and most likely prevent you from climbing the ladder saying insurance reasons will not allow you to go up there. The real reason is the cement hold is 1/2 empty in many cases. That hold is supposed to be full to the top with cement, if it is not then you are NOT getting the right ratio of sand to cement mixture. Even if you are lucky enough to have a true six sack ratio (very hard to come buy), just the nature of how the material is prepared and ends up in your swimming pool is so substandard to Shotcrete.

The material in Gunite is mixed with an auger under the truck with sand and cement. The result dumps into a hopper where pressure is added so the dry material is forced tot he nozzle in your back yard. At that point (this is where it gets funny) the Gunite company then takes your back yard garden hose and tapes (honestly) it to the nozzle and turns your hose garden hose on. This garden hose of yours spraying wildly with dry cement and sand is shot on your pool. Ok, now just use common sense, if we could take the cement and properly mix it with cement ratio's which we get in writing from the shoot truck for each batch, and we use water that we bring with on your own trucks, then the "pre-mixed" wet cement is forced to the pool, don't you think that might just be a better way of doing it? You really don't have to be a rocket scientist to figure this one out, it just is far superior and costs more per yard from the vendor than standard Gunite.

For this reason areas that have a lot of sand and or low soil conditions (such as sand) Arizona, Las Vegas to just name a few, find it impossible see a Gunite company, they all use shotcrete. Simply put shotcrete is the Ferrari of pool cement applications. Rember the cement structure is the most important element of yoru swimming pool, you just might want to think this one through. We us ONLY shotcrete as a standard on EVERY pool we build.

Some interesting reading for you can be found at this link: http://www.shotcrete.org/ASAfaqs.htm

[A couple of related FAQ's from the American Shotcrete Association (http://www.shotcrete.org/ASAfaqs.htm#Q1)]

Question 1: What is the difference between shotcrete and Gunite?

Answer: Shotcrete is an all-inclusive term to describe the spraying of concrete or mortar that may be accomplished through either a dry- or wet-mix process. Gunite refers only to the dry-mix process in which the dry cementitious mixture is blown through a hose to the nozzle, where the water is injected immediately prior to application. Because complete mixing of the water and dry ingredients is not possible in the nozzle, mixing is completed as the material impinges on the receiving surface, through manipulation of the nozzle. This requires a very highly skilled nozzleman, especially in the case of thick or heavily reinforced sections. Large aggregate is seldom used with the dry-mix process. Wet-mix shotcrete involves pumping of a previously prepared mixture, typically ready mixed concrete, to the nozzle. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface. The mixture usually contains minus 1/2 in. aggregate, although larger-size aggregate has also been used.

The use of the term "shotcrete" first occurred in Railroad Age magazine more than 50 years ago in place of the then proprietary word "Gunite," and has been used by the American Concrete Institute since at least 1967 to describe all sprayed concrete or mortar.

Question 33: We are having a pool built with shotcrete. The pool company has asked us to change the contract to allow them to use the wet method instead of the dry method of shotcrete. I have read through your website and found it helpful in understanding the difference between the two, but I would like to know if one is better or more sound than the other.

Answer: Pools are built with both processes. Some find it easier to shoot pools with the wet method. But, when properly done, there should be no difference in performance between wet and dry process shotcrete. Depending on the complexity of the pool, the wet method placement can be faster than the dry method. It comes down to the experience of the contractor and their crews, for a good quality placed pool shell. The nozzleman plays a key role in the placement of well placed shotcrete in both methods. The geographical area may determine the economics of which method is used. Curing of the in-place concrete shell is the same for both processes (water curing for 7 days). Wet concrete has a 90 minute window from the time it is batched at the plant until it placed. Temperature of the material and the air temperature can increase or decrease the set times of the concrete. Typically Ready-Mix companies hold back 10-15 gallons of water in the mix so that the contractor can adjust the slump of the concrete on site. Adding 1 gallon of water over the design mix (amount of gallons of water per yard of concrete) can decrease the strength of the concrete by 200psi.
If you have additional concerns, the following questions should be asked:
*Does the contractor have a good track record of shooting pools with the wet method?
*How many pools have they completed with the wet method?
*Can you provide a list of past completed jobs?
*How do they plan to incorporate the trimmed concrete into the shell? (The rebound and the trimmed concrete play a key role in the final quality of the pool shell.)

*What concrete mix design do they plan to use?

Can someone explain if the following is accurate?

One of the pool builders in my area uses Poured and Formed. He said that when you "shoot" shotcrete that there will always be a gap of space behind the rebars that can lead to problems, by pouring and forming you know you've packed in all the air space...are the spaces a problem in spraying and would a better company take measures to insure that all the spaces get filled?
Tara

Voids behind the steel are only a problem if you don't have a good nozzleman

The only way you know you have filled in all the air spaces is if you were superman and had x-ray vision.

Shotcrete has intentional air has a part of its process of being blown on.....thats why its one of the most porous forms of concrete....kinda like sponge.