Your ears are amazing and intricate structures that capture sounds and convert them into electrical signals that zip into your brain.
Sound is actually a vibrating wave of pressure that travels through the air (or through liquids or solids for that matter).
There are three main parts to the ears: the outer ear (purple in the figure below), the middle ear (green), and the inner ear (blue).

The outer ear is the only part of the ear that I can easily see when I look into your ear. It is the ear canal and the outside surface of the ear drum. The part of your ears that you see acts as a funnel to collect sound waves that pass down to your ear drum (also known as the tympanic membrane).
The sound wave then passes into the middle ear, which is the space behind the ear drum. The ear drum is attached to 3 small bones, the malleus, incus, and stapes. These 3 bones are collectively known as the ossicles. The ossicles conduct the sound waves to the inner ear. Normally, the middle ear space should be filled with air, and the air pressure should be the same as the air outside your body.

The Eustachian tube (at the bottom of the picture) is the connection from the middle ear space to the back of your nose.  Whenever you are on an airplane or otherwise experience changes in air pressure, you can clear pressure out of your ears by yawning which opens the Eustachian tube.

The inner ear is a fluid-filled space surrounded by bone. It is divided into the snail-shaped hearing organ (called the cochlea) and the balance organ which is composed of the semicircular canals among other things. The INNER ear is supposed to be filled with fluid. When doctors talk about “fluid in the ears” as a problem, what they are talking about is fluid in the MIDDLE ear space.
The inner ear collects the sound waves that were transmitted via the eardrum and ossicles. Then tiny cells inside the cochlea convert the sound pressure waves into electrical signals that are passed to the brain via the cochlear nerve. Your brain then perceives those signals as sound.

Problems with any part of the ear can lead to hearing loss. We generally divide hearing loss into two main categories:
Sensorineural hearing loss: caused by problems with the inner ear, cochlear nerve, or brain. i.e. you have hearing loss because you cannot properly perceive the sound.
Conductive hearing loss: caused by problems with the outer ear or middle ear. i.e. you have hearing loss because you cannot get the sound from the outside world into your inner ear efficiently.
We’ll talk more about common diseases and problems that cause hearing loss in a future post.