Obviously referring to hair alone, I'd like to know the differences between the listed words. All dictionaries I checked give pretty much the same definition for all and treat them practically as synonyms. Still, I believe there indeed are minor differences.
A plait, or a braid, is an interwoven length of something - fibre, cloth, hair, etc - made by intertwining three or more separate strands. Ropes are braided, as are steel cables. That is the gist of the definition as I have found in every online dictionary I've checked.
Speaking from experience of British English, there is a difference between the two. When referring to human hair, plait is usually used for a relatively loose construction of three or more strands where each strand has a decent amount of hair, and those strands are clearly visible. In a braid, each strand is generally much thinner, made up of fewer hairs, and they are very tightly wound - so the eventual result is thin and the structure is hard to see without looking very closely. However, this distinction may be different in different dialects. I have certainly known Americans to call the structure that I would call a plait a braid.
A lock of hair is a small piece of hair - several strands kept together. It is particularly used to refer to hair that has been removed from a person's head, and might be kept in a locket or other keepsake. Some people have locks of hair put into custom-built soft toys. However, it is also used to refer to the hair on a person's head - either referring to the hair as a whole as locks, or referring to one stray or isolated bit of hair as a lock, such as "a stray lock of hair fell over her forehead".
A tress is basically like a lock, except it implies longer hair. It is rarely used in the singular, and generally appears in florid descriptions of a person's hair, as in "her long blond tresses fell over her shoulders". It's also an archaic verb meaning "to arrange a woman's hair".