text-string is the type of strings that can hold any character, that is, (vector cl:character length). This is the string type that is guaranteed to always hold any character used in writing text (program text or natural language).
simple-text-string is the simple version of text-string, that is, the string itself is simple. Equivalent to:
(simple-vector cl:character length)
text-string uses 32 bits per character. Applications that use many strings and are very large, when they know they do not use the full Unicode range, can consider using base-string (up to 8 bits) or bmp-string (up to 16 bits) to reduce memory usage.
LispWorks User Guide and Reference Manual - 20 Sep 2017