When I first came to England I was quite confused about all the different terms referring to meals. This is what people explained to me back then:
1) It has got something to do with the time of the day.
"Lunch" refers to a meal in the middle of the day.
"Tea" can refer to a meal in the afternoon or in the (more or less) early evening.
"Afternoon" tea is eaten in the afternoon.
"High tea" is in the early evening (even though many people say "high tea" to afternoon tea now, historically, it is in the early evening).
"Five-o`clock tea" takes place around 5pm and can start as early as 4pm but always ends at 7pm at the latest.
"Supper" refers to a meal in the (late) evening.
2) It has got something to do with whether the meal is cold or hot.
"Dinner" is a hot (cooked) meal. Dinner can be eaten at lunchtime or at teatime. Or, in other words, you can have "dinner" at lunchtime or at teatime as long as it is cooked. You can also have two dinners per day (if you eat a hot meal for lunch and for tea). You can refer to a hot meal at lunchtime as "lunch" but you cannot refer to cold meal at lunchtime as "dinner". Analogously, you can refer to a hot meal in the evening as "tea" but you cannot refer to a cold evening meal as "dinner". However, if you have your evening meal very late, you would refer to it as "supper", regardless of whether it is hot or cold.
3) It has got something to do with what is served.
E.g., "Cream tea" (taken in the early to mid-afternoon) typically consists of tea, scones, clotted cream and jam.
E.g., "High tea" (taken in the early evening) consists of tea, scones, cakes, sweets but also sandwiches and other savory dishes. It is relatively heavy, and there is quite a bit of variety of dishes.
4) It depends at what sort of table you sit when you consume the meal.
"High tea" is taken at a dinner table (i.e. a "high" table).
"Low tea" is taken in a more casual environment, e.g. at a coffee table (i.e. a "low" table).
Lunch can be taken at any table (even the desk!!)
Supper, usually consisting of milk and biscuits or some other light snack, can be eaten in the living room or in bed -- again, there is no table required. An exception is a "warm supper" if there was no time or opportunity to eat dinner earlier on.
"Five o`clock tea" is normally viewed as an afternoon tea and includes both sweet and savory dishes, e.g. scones, cakes, sandwiches. It was traditionally introduced during Victorian times when the main two meals in England were breakfast and dinner and lunch was very frugal. It is said that Queen Victoria had difficulty managing till dinner time after eating very little for lunch so one of her Ladies-in-Waiting came up with the idea to have her eat something in the afternoon.