An amphiphile is a molecule that has both hydrophobic and hydrophilic characteristics.
Part of the molecule is polar and can interact with water molecules, often in this region, there is a partial or complete charge separation. The polar head can have a complete charge in solution (salt), the conditions in which this charge is kept (acidity constant) may change the characteristics and properties of the molecule itself. Quaternary ammonium salts are an example of salts that keep their positive charge independently from the pH of the solution, so, amphiphile with this type of polar head will keep their properties at any pH.
Phosphatidylcholine structure, the polar head is made of the quaternary ammonium unit (blue), and the phosphate group (yellow).  The two tails are made of two fatty acid residues 16C long.

Another part of the molecule is hydrophobic and is usually made of long hydrocarbon chains, like fatty acids (hydrophobic tail), or by aromatic apolar structures, this region interacts with oils or solvents helping to keep them in solution. Detergents, for example, help in solubilizing and removing fats and oil from surfaces.
Depending on the concentrations and conditions of the solutions amphiphiles create more complex structures like double layers (cellular membrane), liposomes, and micelles that are used to keep the oils in stable emulsions.

These molecules are halfway between hydrophobic and hydrophilic molecules and their presence plays important roles in every region of the cell, helping in guiding, solubilizing, and processing different biomolecules.