grep and

# EXAMPLE: displays any lines that contain either "include this string" or "this works too" 
egrep "include this string|this works too" *

# SYNTAX:
# egrep "<pattern-1>|<pattern-2>" *

# egrep is combined with the "|" character (pipeline: normally the shift alternative to the "\" on the keyboard)

With -i flag you can grep a substring with insensitive-case:
grep -i "substring_with_lower_or_upper_case"