R: what are Slots?

Slots are linked to S4 objects. A slot can be seen as a part, element or a “property” of an object. Say you have a car object, then you can have the slots “price”, “number of doors”, “type of engine”, “mileage”.

Internally, that is represented a list. An example :

setClass("Car",representation=representation(
   price = "numeric",
   numberDoors="numeric",
   typeEngine="character",
   mileage="numeric"
))
aCar <- new("Car",price=20000,numberDoors=4,typeEngine="V6",mileage=143)

> aCar
An object of class "Car"
Slot "price":
[1] 20000

Slot "numberDoors":
[1] 4

Slot "typeEngine":
[1] "V6"

Slot "mileage":
[1] 143

Here, price, numberDoors, typeEngine and mileage are slots of the S4 class “Car”. This is a trivial example, in reality slots themselves can be again complex objects.

Slots can be accessed in numerous ways :

> aCar@price
[1] 20000
> slot(aCar,"typeEngine")
[1] "V6"    

or through the construction of a specific method (see extra documentation).

For more on S4 programming see this question. If the concept still sounds vague to you, a general introduction in Object Oriented Programming could help.

PS: Mind the difference with dataframes and lists, where you use $ to access named variables/elements.