Pc case expansion slots

Solution
Are you talking about a PC Case? Or the motherboard?

Expansion slots on the motherboard are used to connect devices like GPUs, Wireless adapters, high end Sound cards, etc.
Your Case (the box your PC goes in) has cutouts on the back which line up with those slots so you secure the add-in cards with screws and have access to the external connects on the back of the card.

A case with "7 slots" will almost certainly be a full sized ATX case, that can fit a standard sized ATX motherboard with all 7 slots available.

Unless you're building some server or mining workstation with very unusual requirements, a 7 slot case with ATX motherboard is completely standard and all you should ever need.
Are you talking about a PC Case? Or the motherboard?

Expansion slots on the motherboard are used to connect devices like GPUs, Wireless adapters, high end Sound cards, etc.
Your Case (the box your PC goes in) has cutouts on the back which line up with those slots so you secure the add-in cards with screws and have access to the external connects on the back of the card.

A case with "7 slots" will almost certainly be a full sized ATX case, that can fit a standard sized ATX motherboard with all 7 slots available.

Unless you're building some server or mining workstation with very unusual requirements, a 7 slot case with ATX motherboard is completely standard and all you should ever need.
 
Solution
It's the motherboard which has the actual expansion slots.

The case just has a certain number of rectangular "apertures" (covered by removable blanking plates) to accommodate the metal spine of whatever expansion cards you install on the motherboard. I suppose most people just call those "apertures" slots, which can confuse some in relation to cases versus motherboards.

Typical uses for the expansion slots are:

Graphics Card
Sound Card
Network Card
USB interface Card (to provide more or faster USB ports)
WiFi Card

etc.
 

Perhaps technically you're right and they shouldn't really be called "slots" on cases, but it's certainly slipped into common usage. Corsair, for example, include "Case Expansion Slots" in the tech-specs table for their cases.