There are two kinds: visible and invisible.
(1) Visible, a sort of impression of the Sacred Wounds of the Saviour, on the feet, hands, side, and forehead. These wounds appear spontaneously, without being provoked by any exterior wounding, and they issue periodically a non-corrupted blood.
(2) Invisible, which cause great suffering without the outward marks. Stigmatization only exists among ecstatics, and is preceded and accompanied by very severe suffering, physical and moral, which renders the subject conformable to Jesus’ suffering; not pathological, but due to the intervention of an intelligent and free cause acting on the stigmatics.
Stigmata are symbols of union with Christ and participation in His martyrdom; found only on persons who practise the most heroic virtues and have a great love of the cross. The first stigmatic known is Saint Francis of Assisi. Venerable Gertrude van der Ousten (died 1358), received the sacred stigmata because of her great devotion to the Sacred Passion. Dr Imbert names 321 stigmatics, in whom there is every reason to believe Divine action. Great precaution is needed in determining whether stigmata be real, apparent, or due to self-deception on the part of the stigmatist.