Sed is a powerful text manipulation tool that can be used to perform various operations on text files and streams. Developed by Lee E. McMahon in 1973, Sed (short for Stream EDitor) is a Unix utility that can be used to search, replace, and transform text in a file or pipeline. In this beginner's guide, we will cover the basics of Sed, including how to write it, provide an example, and explore some of its best applications.

How to Write Sed
Sed is a command-line tool that is typically used in conjunction with other Unix utilities, such as grep and awk. The basic syntax for running Sed is:

```
sed [options] 'command' file
```

where 'command' is a sequence of Sed commands enclosed in quotes and file is the file or files to which the command will be applied. Here are some common Sed commands:

1. Substitute: The substitute command replaces a pattern with a new string. The syntax is `s/old/new/g`, where `old` is the pattern to be replaced, `new` is the replacement string, and `g` means global, i.e., all occurrences of `old` will be replaced.

2. Delete: The delete command deletes lines that match a pattern. The syntax is `/pattern/d`, where `pattern` is the pattern to match.

3. Print: The print command prints the specified lines. The syntax is `n1,n2p`, where `n1` is the starting line number and `n2` is the ending line number.

4. Append: The append command adds new lines to the output. The syntax is `a\`, where `a` is the command to add and `\` tells Sed that the command is on a new line.

Sed Example
To illustrate how Sed works, let's consider the following example. Suppose we have a file called 'data.txt' that contains the following text:

```
Hello, world!
This is a test.
```

If we want to replace the word 'test' with 'example', we can use the following Sed command:

```
sed 's/test/example/g' data.txt
```

The output will be:

```
Hello, world!
This is a example.
```

Best Applications for Sed
Sed is a versatile tool that can be used for a wide range of text manipulation tasks. Here are some of its best applications:

1. Editing configuration files: Sed can be used to modify configuration files by replacing or deleting lines that match certain patterns.

2. Reformatting text: Sed can be used to reformat text by inserting or deleting newlines, adding or removing whitespace, and converting between different text formats.

3. Scripting: Sed can be used to automate repetitive text manipulation tasks by creating scripts that apply Sed commands to multiple files.

4. Regular expressions: Sed supports regular expressions, which are a powerful tool for searching and manipulating text.

Conclusion
Sed is a powerful tool that can be used to perform a wide range of text manipulation tasks. By learning the basics of Sed, you can become more efficient at working with text files and streams. Whether you're editing configuration files, reformatting text, or scripting repetitive tasks, Sed is a versatile tool that can help you get the job done.