| : This is the pipe operator, it serves to pass the output of one process as input to another:
```
foo | bar
```
This runs the program foo and passes its output as input to the program bar.

>,<,>> and <<: These are the redirection operators, they serve to send data to/from files:
- foo > bar : runs the program foo and saves its output to the file bar, overwriting¹ its contents and creating it if it does not exist.
- foo >> bar : runs the program foo and saves its output to the file bar, appending to its contents and creating it if it does not exist.
- foo < bar : runs foo, telling it to read input from the file bar.
- The << is a special case, since there is no point in "appending" input to a command, the << is primarily (exclusively AFAIK) used for Here Documents:
```
$ cat << EOF > file.txt

> Hello World!

> EOF
```
  The construct << SomeStringHere > Out.file will redirect all text written until it encounters the ending string (EOF in the example above) to the target file. Here docs allow you to easily format multi-line strings and include variables and special characters.

The <<< operator, the Here String, is like a Here Document but it expands variables. So, for example:
```
grep foo <<< "$bar"
```
The command above is equivalent to echo "$bar" | grep foo.

What you are actually looking for is called process substitution and is another way to pass the output of a command to another. It consists of <(command).
```
foo <(bar) 
```
So, for your at example, you could do
```
at now < <(echo "notify-send HELLO")
```
The above works because process substitution actually creates a file (read the link above for more details) and it is the file descriptor of that file that is passed with < to at now.

¹ The default behavior is to overwrite, this can be modified by setting the noclobber option to bash. If set, echo foo > bar will fail if bar exists. In that case, it can be forced using echo foo |> bar instead. See section 3.6.2 here.