Aghora is one of the 'vaama-marga' in Hinduism i.e. left-hand-path is considered vaamachaara. The normal Hindu rituals are dakshinachara or colloquially called right-hand path.
Aghoris believe that although dakshinachara is more easier, socially accepted and 'saner' way to attain moksha and elevate kundalini it's painfully slow. Aghoris on the other hand use vaamachara, which is considered very difficult (it's very easy to fall from dharmic path to adharma), socially taboo (the ingredients in a tantric pooja and the yantras etc).
Tantra prescribes something called pancha-makara (5 things starting with 'm'). These include Madya (wine), Mamsa (meat), Matsya (fish), Mudra (cereal), and Maithuna (sexual intercourse). Often it's said that these are allegories for other subtle elements. There are tantric yogic mudras like sahajoli, vajroli mudras which are again related to sex organs.
The reason Tantra is not popular compared to dakshinachara is that it's very easy to stray from the path of moksha and indulge more in the mundane world when surrounded by earthly pleasures including tamasic food, intoxicants and sex.
Since these things are considered taboo and scorned off in dakshinachara, people started concocting stories around people who practice vaamachara including but not limited to Aghoras being necro-cannibals (corpse eaters) and what-not.
For a more elaborate treatment, I recommend a three part book series titled Aghora.