3. How many strings?
The standard bass has four strings tuned to E-A-D-G, which corresponds to the four lowest strings of a guitar. However, bass strings are much thicker, because they are tuned an octave lower. Tuning a bass works exactly like tuning a guitar.
Some advanced players use basses with five or even six strings. This was started in the late 1970's by virtuosos like Anthony Jackson. A five string bass usually has a low B string, while a six string model adds a high C string.
Some individualists even use instruments with seven or more strings. These basses are called ERBs (extended range basses). But no matter how much sense instruments like this may make for certain players—a four string bass is usually sufficient for beginners.
Many legendary bass heroes have played four string basses and still do to this day. Think of bassists like Jaco Pastorius, Jeff Berlin, Marcus Miller, Stanley Clarke, Flea, Steve Harris, Jack Bruce, Chris Squire ... the list goes on and on!