Excel string concatenation (i.e. joining together text strings in Excel) can be performed by either using using the Excel & Operator or using one of the builtin Excel functions (the Concatenate, Concat or Textjoin function).
These methods are each discussed below.
Excel String Concatenation Index: 
Concatenate Using the & Operator 
The Excel Concatenate, Concat and Textjoin Functions 
Concatenate a Date and Time 
Concatenate a Line Break 
Concatenate Quotes 
When used between two text strings, the & Operator returns a single text string, consisting of the concatenation of the two original strings.
The spreadsheet below shows three examples of Excel string concatenation, using the & operator.
Formulas:
 
Results:

Note that the formulas in the above spreadsheet concatenate a space (" ") between the forenames and surnames.
Excel provides three builtin functions to concatenate text strings. These are:
Concatenate    Joins together two or more text strings (Replaced by the Concat function in Excel 2019) 
Concat    Joins together two or more text strings, or arrays of text strings (New in Excel 2019  replaces the Concatenate function) 
Textjoin    Joins together two or more text strings, separated by a delimiter (New in Excel 2019). 
Note that the Concat function has replaced the Concatenate function in Excel 2019 (although the Concatenate function is still available in Excel 2019, for compatibility with older versions of Excel).
The only difference between these two functions is that the Concatenate function can only accept single text strings (or references to single cells containing text), whereas the Concat function can also accept arrays of text strings (or references to arrays of cells containing text).
The syntax of the Concatenate function is:
where the text arguments are two or more text strings that you want to join together.
The syntax of the Concat function is:
where the text arguments are two or more text strings (or arrays of text strings) that you want to join together.
The syntax of the Textjoin function is:
where:
The following spreadsheet shows three examples of Excel string concatenation, using the Concatenate, Concat and Textjoin functions.
Formulas:
 
Results:

Note that, in the above spreadsheet, a space is inserted between the forename and surname of the returned text string. This is supplied to the functions as:
Dates and Times in Excel are stored internally as numbers. It is just the formatting of a cell that causes a number to be displayed as the date and/or time that it represents.
Therefore, if one of the elements of your Excel string concatenation is an Excel cell containing a date or time (or any other formatted number) it is the underlying number, that will be used in your your concatenated string, not the formatted date/time. An example of this is shown in the following spreadsheet:
Incorrect Concatenation of a Text String and a Date
Formula:
 Result (Oops!):

If you want a date or a time (or any other formatted number in Excel) to form a part of your concatenated text string, you need to convert the number to a text string using the required formatting. This can be done using the Excel Text function.
An example of the correct concatenation of text and a date is shown below:
Formula:
 Result (as required!):

Note that the formula in the above example spreadsheet uses the Excel Text function, which receives a value (in this case the date in cell B2), and a formatting definition (in this case, the date format "ddmmmyyyy"), and returns a text string (in this example "29Jan1980").
A common problem that many users encounter, when using Excel string concatenation formulas, is how to insert a line break.
The easiest way to concatenate a line break is to use the Excel Char function. On most computer systems, the integer 10 represents the line break character. Therefore, the function CHAR(10) returns a line break. This is illustrated in the example spreadsheet below:
Formula:
 Results:

Note that, for a line break to be displayed in an Excel cell, the Wrap Text option must be enabled. In recent versions of Excel (2007 and later), this can be controlled by the Wrap Text button, which is located in the 'Alignment' group of the Home tab of the Excel ribbon (see below).
The Wrap Text option can also be controlled from within the Format Cells dialog box. Therefore, if you have an older versions of Excel, which does not have the shortcut button, you can wrap text as follows:
Open the 'Format Cells' dialog box.
(The easiest way to do this is to use the keyboard shortcut Ctrl + 1  i.e. press the Ctrl key and while holding this down press 1).It can be confusing if you want to concatenate quotes in Excel, as Excel will interpret a double quotation mark as the start or end of a text string.
In order to make Excel understand that you require the double quotes to be part of the final concatenated text, you need to use four double quote characters in your concatenation formula. For example:
returns the concatenated string Jim said "Hi".
Alternatively, you can use the Excel Char function to specify the quotes character. On most computer systems, the integer 34 represents the double quotation mark character. Therefore, the function CHAR(34) returns a double quote and the example above becomes:
which again, returns the concatenated string Jim said "Hi".