From the TF2 Wiki:
Bleeding is an attrition damage effect similar to Fire. Currently only the Tribalman's shiv inflicts bleeding, however it is thought more weapons may be released over time with this effect.
Though similar to a Pyro's fire, Bleeding differs in a few key ways
* Bleeding cannot be extinguished. Therefore it cannot be negated by:
o Water
o Jarate
o Compression blast
* Small Medpacks will heal the victim's health but will not stem the Bleeding.
* The indication of Bleeding damage is more subtle than the flame effects of a Pyro's fire.