As one can see in the picture below, in IUI the specialists inserts the sperms directly inside the woman's womb so that they are able to reach the Fallopian tube, where they will meet the egg, by themselves. That is the place where fertilization takes place. After that, the embryo will travel to the uterus and attach there. This technique is the least invasive of all fertility treatments, as fertilization occurs as it would happen naturally.
For IUI to be successful, the patient should have regular menstrual cycles, with no signs of blocked Fallopian tubes. On the other hand, the male contributing the semen sample should have the adequate number of sperms, with good sperm motility.