Canon DSLR’s come in two mounts, EF and EF-S. The EF mount is designed for cameras with a full-frame sensor and the EF-S is designed for cameras with a APS-C sensor.
Unsure of what sensor your camera has? There’s a really easy way to find out: Canon cameras with an APS-C sensor will have both a red dot and a white square near the top of the camera's lens mount, whereas a full-frame camera will have just the red dot. Also, cameras with a full-frame sensor will have a larger surface area covered by the reflection of the viewfinder on the mirror and it will extend almost all the way to the edges of the mount.
If you own one of the following cameras it has an APS-C sensor and can mount either EF or EF-S lenses:
Any EOS Kiss or Rebel model.
And EOS model between 20D and 4000D.
The EOS 7D or 7D MkII.
If you own one of the following cameras it has a full-frame sensor and can only mount EF lenses:
Any EOS 5D or 6D model.
And EOS 1D model.*
*Note: The following cameras are included in the full-frame category, however, they have an APS-H sensor. APS-H sensor cameras have a crop factor of 1.3x but will only accept full-frame EF lenses.
Canon EOS 1D, 1D MkII, 1D MkIIN, 1D MkIII, 1D MkIV.
Let’s look at this the other way, say you’ve got a collection of lenses that you love, have spent time collecting and want to hold on to, but you want to upgrade, replace or purchase an additional Canon body. There's a way to work this out. As with the camera bodies having the white square and/or a red dot on their mount, EF and EF-S lenses have the same markings. At the lower end of the lens barrel, near the mount and typically next to the switches for AF/MF and IS there will be either a white square or a red dot. The white square means the lens will only work on APS-C sensor bodies. The red dot means the lens will work on both APS-C and full frame sensor bodies.
Please note: Owners of a Canon APS-C DSLR camera, like an EOS Rebel, can use lenses designed for full-frame sensors but will experience a crop factor of 1.6x* Owners of a full-frame Canon DSLR, like the Canon 6D MkII, cannot use lenses designed for APS-C cameras.
*Tip: to work out what the new focal length of a lens will be, multiply the focal length by the crop factor. Canon 70-200mm EF lens mounted on a Canon 1300D will have a new focal length of 112-300mm.