This pramana was described by both Charaka and Sushrut.Nyaya,Vedanta and Mimansa also described this Upamana Pramana.
While Jain darshana and Sankhya included this Upamana Pramana as a form of Anumana

Niruktti-

उपमियते अनेन इति उपमानम्/
उपमिती करणम् उपमानम्//
The way through which Upamiti is gained.
Upamiti means knowledge gained through Upama or comparision
Or in other word –The sadhana or instrument or karan through which we get the knowledge.

औपम्यं नाम यदन्येनान्यस्य सदृश्यमधिकृत्य प्रकाशनं /च वि ८/४२
The word Upmana means comparison.There is comparison between to similar things for their superiorty.It gives the idea or knowledge by compairing the objects of similar nature.

सदृश्यमधिकृत्य अन्येन प्रसिद्धेन अन्यस्याप्रसिद्धस्य प्रकाशनम् / चरकोप्स्कार
The knowledge gained by finding similarity of an unknown object with a known one is called ‘Upamaan Pramana.’

प्रसिद्धसाधम्यारत् साध्यसाधनमुपमानम् /
The knowledge of Sadhya is achieved by comparing the features of known object is called as Upamana.

Example-
We gain the knowledge of an unfamiliar ‘gavaya’ by comparing its features with those of a ‘cow.’Since cow is familiar animal,when a person comes across an animal looking like a cow,he will compare the new animal with a cow and come to a conclusion that the new animal is ‘Gavaya.’The conclusion drawn that this must be gavaya is called ‘Upamiti.’

Types-
There are three types of Upamana-
1.Sadharmya Upamana-knowledge unknown object can be gained by compairing it with the similarities of known object
2.Vaidharmya Upamana–knowledge unknown object can be gained by compairing it with the disimilarities of known object
Example-A person can identify a leopard but did not see a lion.Some one tries to explain him in what all ways a lion differs from leopard.Next time when he comes across a lion,he will see all dissimilarities with a leopard and identifies it as a lion.
This will helps in differential diagnosis of diseases like Sandhivata and Amavata,Prameha and Adhoga Raktapitta.
3.Dharmamatropamana-The knowledge of object can perceived by their special character.
Example-Camel can be identified from other animals because of their Hump.
This will helps in diagnosis of disease by their pratyatma lakshana.
Like- Grahani-Muhur baddha muhur drava mala pravrutti.

Clinical Appliance-
1.It helps in identification of hebs
Example-Grudhranakhi-compared with nails of vulture
Gojihva-leaves like tongue of cow
2.Knowledge of disease can obtained by comparison
Example-In Dandaka vyadhi body is stiff like stick.
Dhanustambha-body is like a bow shape
Gidhrasi-gait of patient is same like gait of vulture.
3.Helps in Sharir gyana
Example-Hriday-Pundariken sadrushyam
Aartava-Shasasrukapratimam
4.Upamana pramana used in Dhatu poshana nyaya
Kedarkulya nyaya
5.Use of Upaamana in understanding the benefits of treatment
Example.
शुष्काण्यपी हि काष्ठानि स्नेहस्वेदोपपादनै :/
नमयन्ति यथान्यायं कि पुनर्जीवितो नरान् // च सु १४ /५