In natural conception, the process of fertilisation and embryo growth and development is a series of chemical reactions occurring in a cascade.. each one needing to happen in order for the next one to occur.
The whole process of IVF is really just a matter of working out where in that process there is a blockage or hold up, and 'jumping through that hoop' or bypassing that particular step. I'm over-simplifying really.. but in very basic terms thats what's going on.
If you are not ovulating for example, we might be able to induce ovulation (ovulation induction) or there is a blockage in your tubes, we can 'bypass' that step by going in directly and collecting the eggs, and then bypassing your tubes and inserting the embryo (later) into your uterus.
IVF stands for In-vitro-fertilisation which means Fertilisation in a dish (as oppose to in your body where it would naturally occur). Fertilisation is the process of sperm meeting egg and fusing to form a single cell - which will hopefully then go on to develop into a multi-celled blastocyst and eventually a baby.
To make a baby, you need an egg, you need sperm, they need to be able to meet, fuse (or fertilise) and then the embryo needs somewhere safe and secure to grow (firstly in the lab if you're having IVF, and then in a uterus)
Insemination is the process by which the eggs and sperm are introduced to each other. I think we all know how this happens naturally (intercourse), but in the lab, it can happen in one of two ways: Standard IVF or ICSI
The actual process of fertilisation is still a series of chemical reactions, but by doing ICSI, we bypass some of them.
Standard IVF is when we wash and prepare the sperm and then place them together with the egg in a dish in the incubator overnight and let them do their own thing.
ICSI is where we inject a single sperm into each egg and then put them in the incubator overnight to do all the actual fertilising.
Another time with word 'Insemination' is used, is when you are having IUI or Intra-uterine-insemination. This is when the semen sample is washed and prepared and then a concentrated portion is introduced to your uterus using a small catheter. Placing clean, motile sperm as close as possible to where the egg will be. This is timed to coincide with ovulation which is determined using ultrasounds and sometimes blood tests.