What are bash scripts: Bash scripts are shell scripts written for use with bash
Ok so what are shell scripts: Shell programs interpret comands which are typed in by the user or read out of a file
And what is bash: Bash is a type of shell which my system is automatically set up to use. When I type into my terminal that is what I am using. There are other types of shell (like there are other languages, etc) but bash is good (and all I know so far)
$ i=1; forfile in *.jpg; do mv "$file" $i.jpg; i=$((i+1)); done;
This changes the names of all the files which end .jpg to numbers from 1.jpg upwards.