A star start off as clouds of dust and gas before something starts the process of turning that nebula into a star. A possible cause will be a supernova explosion nearby. Our Sun would have been an area of dust and cloud before something kick-starting the process. Lets just say for the moment, the event was a supernova. The cloud would begin to collapse and coalese and start off as a protostar before moving on.
A Protostar is the first stage when the gas and dust start to coalesce. Both Protostars and T-Tauri stars will be large stars as they collapse under gravitational forces. They will not yet be stars in their own rights yet.
The difference between a Protostar and a T-Tauri star is that a T-Tauri will start to blow away the gas and dust that is surrounding it. Caltech The clouds might contain Lithium which when the star becomes a main sequence, the lithium is usually destroyed. Lithium is a sign of youth. A.A.V.S.O.
T-Tauri stars are young stars, not much older than about 10 million years old. Their mass is usually less than three solar masses. Not all stars will become T-Tauri stars, it is possible our star was once a T-Tauri star as it is conceivable that our star was once a T-Tauri star before becoming a Main Sequence star.
A Main Sequence Star is when a star is converting hydrogen into helium or carbon which is what our star is currently in. A T-Tauri star can be in this phase of its life for over a hundred million years before it has enough mass to begin the main sequence.
A T-Tauri's heat is not generated by conversion but by the continual collapse of the area. Only when the star has sufficient mass to begin conversion that the next phase begins.
T-Tauri was first discovered by John Russell Hind who was studying the night sky when he discovered a star that was no marked on any maps. This star was T-Tauri. Nearby the star is the Hind Nebula, NGC 1555 whose brightness is linked to the variability of the T Tauri star. (A.A.V.S.O.)
T-Tauri stars can rotate rapidly such as a day. For comparison, our star, the Sun does a full rotation in 30 days.
CI Tauri, also in the same constellation is T-Tauri class star that has an exoplanet in orbit. At the same time the star is transforming into a Main Sequence, the clouds and dust that are being blown away are coalescing and forming planets.
There are two types of T-Tauri stars, Classic which have extensive disks and strong emission lines. The other is Weak-lined which are stars that have a weak or no longer have a disk. David Darling
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