According to biblatex doc (p.80-81) :
\Parencite is similar to \parencite but capitalizes the name prefix of the first name in the citation if the useprefix option is enabled, provided that there is a name prefix and the citation style prints any name at all.
\Textcite is similar to \textcite but capitalizes the name prefix of the first name in the citation if the useprefix option is enabled, provided that there is a name prefix.
My problem is that \parencite and \textcite capitalize the author name by default. Therefore, there is no difference between these commands (\textcite has the same behavior than \Textcite). I am using authoryear-comp style.
It it a big problem for me because I cannot cite using \textcite. Indeed it will produces something like "according to NAME (2008), there is ...". The capitalized author name is ugly.
MWE : 
\RequirePackage{filecontents}
\begin{filecontents}{\jobname.bib}
@Book{author00:_title,
author = {Author},
title = {Title},
publisher = {Publisher},
year = 2000}
\end{filecontents}
\documentclass[12pt,twoside,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[english,french]{babel}
\usepackage{lmodern}
\usepackage{xspace}
\usepackage[autostyle=true]{csquotes}
\usepackage[style=authoryear-comp,
hyperref,
backend=biber,
isbn=false,
doi=false,
url=false,
]{biblatex}
\AtEveryBibitem{\clearlist{language}}
\renewcommand*{\newunitpunct}{\addcomma\space}
\addbibresource{\jobname.bib}
\usepackage[colorlinks]{hyperref}
\begin{document}
\noindent \verb+\parencite{author00:_title}+ : \parencite{author00:_title}\\
\verb+\Parencite{author00:_title}+ : \Parencite{author00:_title}\\
\verb+\textcite{author00:_title}+ : \textcite{author00:_title}\\
\verb+\Textcite{author00:_title}+ : \Textcite{author00:_title}
%\printbibliography
\end{document}
useprefix=trueis given in the options,\textcitewill print “van Beethoven”, while\Textcitewill print “Van Beethoven”. – egreg Jan 16 '14 at 16:31french. Inside the language specific declaration you will find:\protected\def\mkbibnamelast#1{% \textsc{\textnohyphenation{#1}}}%– Marco Daniel Jan 16 '14 at 16:32filecontentsto have its effect, you have to load it before the environment; it's better to use\jobnameas a name in example code, so it won't clutter any other file; last,\addbibresourceis to be preferred to\bibliography(the.bibextension is necessary). – egreg Jan 16 '14 at 16:35french;-) or http://tex.stackexchange.com/questions/53309/changing-default-citet-font-in-biblatex – Marco Daniel Jan 16 '14 at 16:35\bibliographyinstead of\addbibresourcebecause I cite with reftex (C-c [) which does not understand it (error : no valid bibliography). – ppr Jan 16 '14 at 16:42