Your voice is an instrument. For example, in a string quartet consisting of four members, two violinists, a violist, and a cellist, each musician plays a member of the violin family of stringed instruments. These instruments differ in size and range, i.e., how low or high they play. Voices are like this, except that the voice is hidden from direct observation, and thus there can be a disagreement about what type of voice a given singer is actually "playing".
This makes a big difference at the training stage of a singer's career. For instance, a baritone with a solid A natural can sing all the baritone arias comfortably, whereas a tenor with a solid A natural and nothing usable above it is severly limited. This creates an incentive for unscrupulous or ignorant voice teachers to misdiagnose a beginning student.
How can you tell what type of voice you have? I have always found men's voices easier to diagnose than women's, especially since men's voices are usually reflected in their speaking voice to some degree. The key is your highest note, if you are an untrained singer. This note need not be particularly comfortable or beautiful. Let me give you the possibilities for men, together with this key note, essentially the top of your middle range.
Now for female voices. Because many musical concepts were codified during the age of enlightment, which corresponds in music to the classical period, rationality prevailed and it was assumed that women's voices were like men's only an octave higher. Nothing could be farther from the truth. Women, even big mezzos and contraltos, often have many naturally occuring high notes, but the passage from the low register to the high may be very difficult. Men, on the other hand, will simply go higher by pushing and shoving until they can go no further. There are three main voice types for women as follows:
In general, absent true evidence to the contrary, if you are a man, assume you are a tenor, if a woman assume you are a soprano. Singing above your true range makes your voice better, whereas singing below it tears it down and blows the color out.