Water molecules are v-shaped and highly polar. The strong hydrogen bonding between molecules gives water some important properties:

• a strong surface tension
• high latent heat (energy required to change state) and specific heat capacity (energy required to change temperature)
• forms a hexagonal lattice as it freezes, causing it to expand (becomes less dense, so ice floats on water)
• dissolves many ionic salts very effectively, as its polar ends allow it to attract and surround ions

Salts of sodium (Na⁺), potassium (K⁺), ammonium (NH₄⁺) and nitrate (NO₃^-) are always soluble in water.

YouTube video about dissociation here.

Acids are chemicals that donate protons (H⁺). In solution hydrogen ions attach to nearby water (H₂O), so they can equivalently be written as hydronium ions (H₃O⁺).
Bases are chemicals that accept (take) protons. Ammonia (NH₃) for example has a spare unbonded electron pair, so it can accept a proton to become ammonium (NH₄⁺).

Liquid water molecules are constantly breaking up and reforming according to this equilibrium: H₂O ⇌ H⁺ + OH^-
This means that removing OH^- is effectively the same as adding H⁺, and removing H⁺ is effectively the same as adding OH^-.

The acidity (concentration of H⁺) or alkalinity (concentration of OH^-) of water relates to the pH of water.
pH is defined on a scale from 0 to 14, where 0 is the most acidic and 14 is the most alkaline (pOH is the exact opposite).
pH = -log[H⁺]   ,   pOH = -log[OH^-]   ,   and   pH + pOH = 14