This statement. although 25+ years old, is still pretty good.

If you take a whole bunch of things that people have to learn to do (e.g. maths, spelling, logic, speed of thought, and memory), they all correlate pretty well together. So psychologists call this statistical manifold "g" for "general intelligence".

Theories like Gardner's multiple intelligences are frankly nonsensical because it ignores statistical reality, and replaces the word "talent" or "ability" with "intelligence". Thus he refers to intelligent (meaning good) dancers with a straight face.

The biological basis of intelligence is largely unknown, although brain size, number of neurons, and amount of folding is correlated.

Cognitive abilities like how much you can hold in your working memory, and how quickly you can make decisions, although correlate and plausibly cause intelligence differences.