Water is composed of an oxygen atom and two hydrogen atoms, fused together through covalent bonding. The unique circumstances of this bonding results in a dipole water molecule, dipole meaning that the molecule has a distinct positive and negative region. The water molecule's distinct composition gives it several unique properties which allow it to easily serve several purposes in Biology. Because it is a polar molecule, it allows other polar molecules to easily dissolve into it as well as interacting with water. Likewise, non-polar molecules don't interact easily with water will attempt to avoid it when possible, clustering together to avoid contact with water. These types of molecules are called hydrophilic (water loving) and hydrophobic (water fearing).
Because of water being a polar molecule, it is also easily able to make hydrogen bonds within it's self. Because water molecules are able to attach to one another, water gains certain cohesive properties which allows for water's surface tension. Water's surface tension is what allows water to support the weight of small insects without the insect breaking through the surface and falling into the water. And because water molecules easily form hydrogen bonds, they can also form hydrogen bonds with other atoms. This gives water adhesive properties, which allows it to attach to other objects.