This is the Tagalog equivalent of the English word who.  It is always followed by an ANG-clause (a focused clause). The expected response to this question is a sentence that has ANG-phrases in both predicate and subject positions (referred to as an identificational sentence). Or, phrased differently, the answer to a SINO question will have both the subject and the predicate in focus. Let's look at the first example:

Sino ang bagong titser?

Si Ginang Santos (ang bagong titser).

Notice that both Ginang Santos and bagong titser are in focus. The direct answer to the question is Si Ginang Santos. The second part is optional, it merely makes the answer more complete. When translating this sentence into English, we see that there is some sort of implied is between Si Ginang Santos and ang bagaong titser. The word is functions like an equal sign. So, if the two parts of the sentence are equal and if the first part of the sentence is in focus, it seems reasonable that the second part of the sentence would be in focus as well.

When the anticipated response is a plural noun, the question word used by many speakers is sinu-sino. Or in other words, if only one person is being asked about, the word sino is used. If more than one person is being asked about then sinu-sino is used. Here are some examples:

Sino ang bagong titser?
Si Ginang Santos (ang bagong titser).

Sino ang kasama mo sa Bacolod?
Si Robert (ang kasama ko sa Bacolod).

Sino ang nagluto ng pansit?
Ang nanay ni Fe (ang nagluto ng pansit).

Sino ang magdadala ng radyo?
Si Noel (ang magdadala ng radyo).

Sinu-sino ang sasama sa Sagada?
Kaming lahat (ang sasama sa Sagada).

Sinu-sino ang taga-Maynila rito?
Si Lita, si Fe at ako (ang taga-Maynila).

Who is the new teacher?
Mrs. Santos (is the new teacher).

Who went with you to Bacolod?
Robert (went to Bacolod with me).

Who cooked the pansit?
Fe's mom (cooked the pansit).

Who is going to bring a radio?
Noel (is going to bring a radio).

Who are going (with me) to Sagada?
All of us (are going to Sagada).

Who are from Manila here?
Lita, Fe , and I (are from Manila).