The five prohibitions:[1]

The verse[2] states that on Yom Kippur one is required to oppress himself. The Sages derived from this verse five oppressions that one must restrict himself from on Yom Kippur, in order to decrease one’s pleasure. All these five prohibitions are of Biblical status, as they are deduced from the above verse.[3]

From when: All the prohibitions apply starting from sunset of Erev Yom Kippur.[4] When one stops doing Melacha, and begins the oppressions from sunset, he has also fulfilled the Mitzvah of Tosefes Yom Kippur.[5]

The following are the prohibitions:

1. Not to eat or drink
2. Not to bathe
3. Not to anoint
4. Not to wear leather shoes
5. Not to have marital relations

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[1] Admur 611/1-2

[2] Vayikra 23:27

^[3] Nevertheless, amongst the above prohibitions, the prohibition against eating or drinking is explicitly mentioned in the verse “And you shall oppress your souls” and therefore carries with it the penalty of Kareis.

Opinion of Rambam: The Rambam learns that the oppressions are derived from the verse which states that Yom Kippur is a “Shabbos Shabboson”, which teaches that on Yom Kippur there is a double rest. Meaning, that besides for resting from Melacha, we are also commanded to rest from different forms of pleasure, as mentioned above. However, Admur 613 writes that we learn it from the verse of  “And the soul which will not oppress itself…”  ע“וצ

[4] See Admur 611/1 and 608/1-2 that all the laws both of fasting and oppression begin from the time of Tosefes Yom Kippur. In Seder Hachnasos Shabbos Admur explains that in truth the real sunset is about 4 minutes later than the visible sunset. The Ketzos Hashulchan [93 footnote 1] learns from this that according to Admur one fulfills the Mitzvah of Tosefes by stopping Melacha from sunset.

[5] Ketzos Hashulchan 93 footnote 1