The formula for water is H2O. H is the symbol for the element hydrogen. O is the symbol for the element oxygen. Subscripts (the small numbers) indicate how many atoms of each element to the LEFT of that subscript there are. If there is no subscript, then it is understood that there is only one atom of that element.
Therefore, the formula H2O implies that there are 2 hydrogens and one oxygen atom.
Oxygen is much more electronegative (has a higher affinity for electrons) than is hydrogen. Therefore, it pulls the electrons towards it more than the hydrogen does. This pull causes the oxygen to be slightly negative and the hydrogen to be slightly positive. We call this unequal distribution of charges within a molecule polarity. Water's polarity relates to its unique characteristics such as having a high specific heat capacity, a strong surface tension, a high boiling point, etc.