A lorry is a large, flat bed truck, covered or open, for carrying goods. The word was in use for certain kinds of freight carrying rail cars long before motor vehicles were invented. The word has been around since the early 19th century and some dictionaries suggest it is related to an English dialect word that means to pull or drag. The bottom line, though, is that nobody really knows.
Confusion arises because the British are often much more specific in the way they use language. Many vehicles that North Americans call "trucks" would never be called lorries in the UK.
For one thing, the cab on a lorry is always physically distinct (though not always separate) from the goods carrying part of the vehicle. If it isn't, it would be called a van. And if it carries liquid - like gasoline or milk - it's a tanker (not a tanker truck).
If the cab of a lorry can be separated from the goods carrying part, that vehicle is called an articulated lorry. The North American truck, often called a "semi", that pulls shipping containers or other large dry cargo containers on a flatbed fitted with wheels is an articulated lorry, or, sometimes, a juggernaut.
And what about a pickup-truck? That's easy. We don't really have those over here in the UK. If you happen to see one, you can call it a pickup truck.