Before us is Mishnah 7 in the first chapter of Maseches Megillah. This Mishnah teaches us three distinctions between different areas in matters of tumah: between a zav who had two sightings and a zav who had three, between a quarantined metzora and a confirmed metzora, and between one who becomes pure after quarantine and one who becomes pure after confirmation.
The law of the zav:
A zav is a male from whom a discharge emerged that is not semen, but a different form of emission. His status changes according to the number of sightings:
One sighting - his status is like one who had a seminal emission: he immerses in a mikvah and becomes pure that same night.
Two sightings - he is tamei, and he requires seven clean days during which he has no discharge, and only afterward does he become pure.
Three sightings - he requires seven clean days, and in addition he must bring a korban at the end of the seven clean days.
About this the Mishnah says: "Ein bein zav haroeh shtei re'iyos laroeh shalosh ela korban" - the only distinction is that one who had three sightings is obligated to bring a korban, while one who had two is exempt from it. But in all other laws the two are equal: both are on the same level of tumah, and both require seven clean days in order to become pure.
A quarantined metzora and a confirmed metzora:
The next law deals with a metzora, meaning one on whose skin a sign of tzaraas appeared, such as a white spot (and of course this is not the leprosy familiar to us today). If the affliction has no signs of tumah - such as two white hairs, or michyah, which is healthy flesh in the middle of the affliction - the kohen sends him into quarantine in his home for seven days. At the end of those seven days the kohen returns and examines whether a sign of tumah has appeared on him. If a sign of tumah has appeared, he becomes a confirmed metzora, meaning a definite metzora. In any case, both of them are tamei.
The Mishnah says: "Ein bein metzora musgar l'metzora muchlat ela peri'ah u'ferimah" - there is no difference between a metzora who was quarantined for seven days in order to clarify whether he has a sign of tumah, and a definite metzora on whom a sign of tumah appeared, except for these two laws, which apply to the confirmed metzora alone:
Peri'ah - the Torah says regarding the confirmed metzora that his head shall be unshorn, meaning he may not cut his hair but must leave it grown wild.
Perimah - that his garments shall be rent, meaning torn.
One who becomes pure after quarantine and one who becomes pure after confirmation:
The third law also deals with the difference between a quarantined metzora and a confirmed metzora, but not with their conduct while they are metzoraim, rather with the time when they complete their tumah and become pure. In the words of the Mishnah: "Ein bein tahor mitoch hesger l'tahor mitoch hechlet ela tiglachas v'tziporim" - there is no difference between one who becomes pure after having been a quarantined metzora and one who becomes pure after having been a confirmed metzora, except for these two things:
Tiglachas - the obligation to shave off all the hair of the body, which applies to one who becomes pure after confirmation.
Tziporim - bringing the two birds described in the section of the metzora: one of them is slaughtered, and the second is dipped in water containing the blood of the first and then sent away.
This entire process applies only to one who becomes pure after confirmation, and not to one who becomes pure after quarantine.
It is worth noting that there are additional differences between the two: the korbanos brought at a later stage also apply to the confirmed metzora alone and not to the quarantined one. However, our Mishnah does not enter into those later korbanos, since it deals only with the things that take place at the time the metzora initially becomes pure, and not with the things that come afterward.
In summary: In this Mishnah we learned three distinctions in matters of tumah: between a zav who had two sightings and one who had three, where the entire difference between them is the obligation of a korban alone; between a quarantined metzora and a confirmed metzora, where the entire difference between them is peri'ah and perimah; and between one who becomes pure after quarantine and one who becomes pure after confirmation, where the entire difference between them is tiglachas and tziporim.