In more detail
The copy is made through a write blocker so the original cannot change, and a Hash of the original and the image are compared to prove they are identical. Analysts then work on the image, leaving the original untouched, which protects the Chain of custody. Common formats include raw dd files and E01. Unlike a file backup, a forensic image copies every sector, including unallocated space, slack and deleted data, which is where File carving finds its material. E01 adds compression, case notes and built-in checksums; raw images are simply the bytes.
Why it matters
If the original is modified, even by mounting it on a normal computer, its hashes and timestamps change and the evidence can be challenged. Imaging also has traps: unreadable sectors must be logged, and some options alter the output. With dd, conv=noerror,sync pads a short final read up to the block size, so if the device size is not a multiple of bs the image gains extra zero bytes and its hash no longer matches the source.
Example
Imaging a 1 MiB test source and verifying the copy:
$ dd if=disk.raw of=evidence.dd bs=64K conv=noerror,sync status=none
$ sha256sum disk.raw evidence.dd
30e14955ebf1352266dc2ff8067e68104607e750abb9d3b36582b8af909fcb58 disk.raw
30e14955ebf1352266dc2ff8067e68104607e750abb9d3b36582b8af909fcb58 evidence.dd
The same command with bs=4M produced a 4,194,304-byte image whose hash (bb9f8df6...) did not match. Verify image hashes with the File Checksum Calculator.