The common bar line is represented with a pipe symbol, |.
Here’s an example, 2 notes, a bar and two other notes:
X:1
CD | EF
Double bar can are formed with a combination of pipe (|) and square brackets ([ or ]).
|] thin-thick double bar line
|| thin-thin double bar line
[| thick-thin double bar line
Here’s how it looks like in context:
X: 1
C2 [| D2 || E2 |]
Of course bars are also used to indicate repeated parts. In this case the colon symbol is used:
|: start of repeated section
:|. end of repeated section
:: start & end of two repeated sections
1[ and 2[ are used to represent first and second repeat.
When adjacent to other bar symbols, they can be shortened to |1 and :|2.
1[ first repeat
2[ second repeat
|1 bar and first repeat
:|2 bar and second repeat
Here is a simple example:
X: 1
|: CD EF | FE |1 DC :|2 CC :|
For a more detailed explanation of bars and repeats, see the offical ABC website.