I am not going to give you pages of the jargon filled technical speak on Hypnosis, the internet has plenty of those articles if you want to find them. But a brief explanation is required.
The usual Hypnosis definition is :
a relaxed trance like state of altered consciousness, similar to sleep but not actual sleep, where a person is highly receptive to suggestion.
I want you to think of your mind as having three layers,
The Unconscious mind.
The Subconscious mind.
The Conscious mind.

The unconscious mind can be considered as that part that controls the body functions such as breathing, heartbeat, cell growth, etc. and fundamentally cannot be changed. You can breathe faster or slower but you still breathe.
The conscious mind is the part that you use daily to make decisions and interact with people and the world around you. It is logical and contains judgement, reason, short term memory, will power. It is the part that filters your daily information and decides if it likes it or not, if it is good or bad, if it is desirable or not.
The subconscious mind is the part we deal with in hypnotic trance. This is the part where emotions and feelings, (positive and negative), long term memory, forgotten memories, fears, confidence, motivation, habits are stored. It contains the infamous "flight or fight" tactic. It is reactive and irrational.
The subconscious does not know what is real or what is fiction, it needs the conscious mind to decide that through what is called the "critical factor". Your critical factor is essentially how your thoughts and emotions navigate your interpretation of your world, so events, situations and suggestions trigger certain emotional responses based on that interpretation. This can be different from person to person, hence some people are afraid of spiders, for example, while others are not.
Consider the subconscious as the core of your personality and store of life experiences. It contains the good and the bad, the useful and obstructive parts of you.
In hypnotic trance we by-pass the conscious mind in order to work with the subconscious mind without that judgmental conscious part blocking out suggestion. That is why hypnosis works so effectively, because in dealing with the core personality traits we can work at changing the core negative and obstructive parts and create new positive, desirable outcomes and attributes.
Perhaps then, a better way to define hypnotic trance is :
the distraction of the conscious mind so that we can communicate with the subconscious mind directly.
What is Hypnotherapy then?
Hypnotherapy is the use of hypnotic trance by a hypnotist to aid a client in making positive, permanent, desirable changes that they could never make, or certainly not easily make, on their own through willpower alone.
Note the word "desirable". Despite some of the myths that surround hypnosis, no-one can be made to do something against their will through hypnotic trance. Hypnotherapy works through the clients trust in the hypnotist, never by the hypnotist's "control" of the client and that is the simple truth of it.
And anyone who has ever used willpower and conscious thought to change negative aspects of their life or remove fears will know how difficult it can be or at least how short term the results are at least. Even trying to enhance our positive thoughts and motivation through willpower and conscious thought can be very difficult to maintain over long duration.
For example the smoker who keeps trying to quit but has to try twenty times before getting anywhere and then always craving the next puff.
How many annual gym memberships go unused after only three or four months of exuberance?
How many panic attack sufferers spend their time worrying about relapse despite medication and years of avoidance coping strategies?

Hypnotherapy Vs .................
Research shows time and time again that hypnotherapy works. The reality is other talk therapies work also.
But studies show that no other talk therapy works as fast, permanently or as easily as hypnotherapy.
Some talk therapies can take months, maybe even years, before any real change or habit alteration occurs. In many cases no change occurs at all. The therapy simply provides the sufferer with coping or avoidance strategies that require constant monitoring, putting limitations on the sufferer as to what they can or cannot do for fear that they could trigger a relapse.
Because hypnotherapy deals directly with the core issue rather than the symptoms then changes are far more permanent and immediate.
Can you be hypnotised?
The answer is yes. Everybody can be hypnotized. Equally, everybody can resist being hypnotized if they so choose. They just have to let their conscious mind do the thinking for them.
In fact everybody experiences hypnotic trance frequently all the time, they just don't realize it. All those times you are day dreaming just drifting along in your world. Someone talking to you and you don't hear them, that "tuning out" sensation. That is essentially hypnotic trance - the conscious mind being distracted while old memories or nostalgia or even nothing at all, drifts through our mind and we are left wondering, hmmm? where did that come from?
So essentially, all hypnosis is self hypnosis. It depends on the client to allow hypnotic state to be achieved. A skilled hypnotherapist is acting as a guide for the client to firstly achieve the trance state and secondly identify where in the subconscious store of emotions and memories the cause(s) of the issue or concern or habit are kept and then accommodate the client's desired changes and improvements. Allowing the client to understand the original learning interpretation and source. Then through suggestion and guidance allow that client to re-learn the interpretation turning it into a rational experience rather than attaching negative emotion to it. Removing the limits and doubts essentially.