Most search engines will recognise an asterisk (*) as meaning any number of any characters.  

placent* 

will find placenta, placentae, placental and placentas.

Consider the following statements:

• A plant stem includes many specialised cells including xylem and phloem cells for the transport of water and nutrients.

• Putting this in more biological terms, stem cells have the unique and defining characteristics of self - renewal and of differentiation into multiple cell types.

A search for stem cells would locate both statements.

If you are searching for information about stem cells you can force the search engine to look for stem cells as a phrase by enclosing it in inverted commas (").

A search for "stem cells" would only locate the second statement.

Enclosing a group of terms in parentheses () will force a search engine to process this part of the search before it process another part.

Parentheses can be nested, that is, you can include a set of parentheses within another set.

For example:

 term1 term 2 (term3 term4 term5 (term6 term7) term8)

will force the search engine to process terms 6 and 7 before processing the result of this search along with terms 3,4,5 and 8 before finally combining the result of this search with terms 1 and 2.

This is important when you apply boolean logic but, more of this later.

The use of asterisks and inverted commas is recognised by almost all databases.

However, individual databases may include other useful punctuation symbols.

For example, a question mark (?) may stand for one or no characters.  This can be useful if American and English spelling differences are a problem.

A search for behavio?r will locate both behaviour and behavior.

Or, in the Web of Science database you can use a NEAR operator.  salmon NEAR/7 virus will locate records where the words salmon and virus are within 7 words of each other.

You should look at the help pages for each database you use for my guidance.