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Cataract is a clouding of the eye's lens. When we look at something, light rays travel into our eye through the pupil and are focused through the lens onto the retina, a layer of light-sensitive cells at the back of the eye. The lens must be clear in order to focus light properly onto the retina. If the lens has become cloudy, this is called a cataract.
Reference: What Are Cataracts?

A chalazion and a stye are both lumps in or along the edge of an eyelid. In some situations it may be difficult to distinguish between a chalazion and stye. A stye often appears as a red, sore lump near the edge of the eyelid, caused by an infected eyelash follicle. When a stye occurs inside or under the eyelid, it is called an internal hordeolum (pronounced "hor-dee-OH-lum").
Reference: What Are Chalazia and Styes?

Conjunctivitis is the term used to describe swelling (inflammation) of the conjunctiva — the thin, filmy membrane that covers the inside of your eyelids and the white part of your eye (known as the sclera). Often this condition is called "pink eye."
Reference: Conjunctivitis: What Is Pink Eye?