From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Absorption (吸収, Kyūshū) is a supportive technique used by certain characters. The user uses it against a person whose strength and abilities they wish to possess. This usually involves the attacker using a part of their body to suck a victim into their body, thus gaining their power and abilities.
Majin Boo's Absorption
Buu fully engulfs and takes an opponent into his body, causing an increase in his physical and/or mental powers. Buu usually has a severed body part liquefy, or he has a piece of skin fall off of his body. The goo will then sneak up behind the person and stretch itself to be large enough to accommodate the target. It will then leap onto the person and try to smother as much of the target as it can on the first strike to make the capture easier. If Buu is lucky, it will completely cover the person directly on the first leap. Once the goo is on the person, it will quickly cover up any part of the person that was not covered when it first leaped. Once the target is completely covered, the goo will squeeze and solidify, trapping the person. It is seemingly impossible to escape it no matter what the circumstances (such as Piccolo, who the goo took a long time to cover due to his armor) with the possible exception of Vegito who allowed himself to be absorbed.
Buu will then raise his finger up, and the goo will then launch up into the air with the person inside of it. The goo will then fly straight into Buu and cover him. During the transformation, Buu will become liquefied himself, meaning that he takes over the goo and has surrounded the person with his own liquefied body. He then begins to mold back into solid form, making sure the person is in the center of the goo. The person is shrunken and is put inside Buu's body in a pod. The transformation is long and usually accompanied by loud moans and groans, most prominently heard when Buu absorbs Gotenks and Piccolo. As Buu is absorbing those two, we can clearly see the Fusion vest forming roughly on Buu's shoulder showing that Buu is using Gotenks as his main power source and whether these sounds are of Buu struggling to absorb two beings at once or just because of all the power that Gotenks has. Many fans speculated that Majin Buu will absorb Gotenks as he is introduced earlier in the saga and most villains have bulgy kind of clothes when they transform and this fits Gotenks' vest very well.
Unlike Cell, who took in life essence only through his tail, Buu can perform this technique with any part of his body, even a severed limb, or he can simply liquefy his whole body and cover the victim with himself. When absorbing another being, Buu always takes qualities of their outward appearance as well. In the case of the Z Fighters, he always donned a garment of his strongest victim (first Gotenks' vest, followed by Piccolo's cape, and finally the top of Gohan's's martial arts uniform). He also gains knowledge of their attacks, like Piccolo's Special Beam Cannon, Gotenks' Galactic Donut and Super Ghost Kamikaze Attack, and Gohan's Kamehameha. He also apparently possesses the ability to use attacks after seeing it done once, as he does so with the Kamehameha, as well as the Shunkan Idô when in Kid Buu form.
The first method was called Seitai Enerugii Kyūshū (生体 エネルギー 吸収, Seitai Enerugii Kyūshū) where he stabbed his target with the sharp end of his tail. The tail would then extract all of the organic matter of the victim, leaving the clothing behind. (The process appears quite agonizing for the victim). According to Future Cell, this is how he fed as he often licked his lips before consuming a large population or an android. He also stated that the more he ate, the hungrier and stronger he became. While this form of Absorption was unusable on partially mechanical beings and was also much less versatile in comparison to Majin Boo's Absorption technique, Future Cell's Absorption via stabbing the victim with his tail was the only form of Absorption used in the series that made the victim completely unable to be freed due to the victim being broken down into genetic material and then integrated directly into his body. During his first confrontation with Piccolo, acid dripped from his stinger, indicating that Absorption happened by injecting digestive Enzymes into the victim and absorbing the dissolving genetic extract.
In addition to the above variation, Future Cell had a second method of Absorption in which he will absorb a person whole. Future Cell will open up the pointed end of his stinger, and it will become a giant sucker. Future Cell will bring the sucker down on the person, and the sucker will suck them through Future Cell's tail into his body. The person will be held unconscious in Future Cell's body. This method was designed for Android 17 and Android 18, who, due to not being completely organic (as they were human before Gero modified them), had to be absorbed whole. This was Future Cell's preferred method, as it gives him full power, and is the only way for him to advance to his Semi-Perfect form and Perfect form, respectively, attained by absorbing one and both of the cyborgs. After Future Cell's self-destruction, it was revealed that his cells had remembered the perfect form that he had achieved by absorbing Androids 17 and 18, and even though he no longer had the androids inside of him, he was able to regenerate into his final, Super Perfect form. Even though Future Cell's tail retracted after becoming Super Perfect Cell, it is shown in Dragon Ball GT that Future Cell can still bring his stinger out and absorb other people using this technique, though Goku was able to escape when Future Cell absorbed him. The tail, like any part of his body, can also regenerate when removed, making it impossible for him to be deprived of his Absorption capabilities without him or his target being killed first.
Janemba has the ability to absorb lifeforms as seen in Dragon Ball Z: Shin Budokai. After Gohan fought and defeated Majin Vegeta, Janemba went in and started to absorb him. The Absorption seemed to be a very painful one, as Vegeta was screaming at the top of his lungs the entire time. It is also possible that the Absorption of Janemba provides his victims with some energy, as when the player engages in the next battle, Vegeta's health is filled to the brink.
Hirudegarn's method of Absorption is to raise his gigantic tail and crush and trap his victims in it. He then sucks their energy out of their body and into his tail up into his head, while growing stronger with each second.
Lood absorbs the people he turned into dolls in order to collect their energy. To do so, he creates winds that bring the doll to his mouth. In the arcade game Dragon Ball Heroes, Luud instead fires a mouth blast that capture his victim and brings him/her to his mouth.