How can I mix itemize and tabular environments?

You can do it like this (\textbullet inserts the bullet, and you have to mark the new row with a &. To be more flexible you can use \labelitemi instead of \textbullet as Juri said in his comment):

%http://tex.stackexchange.com/questions/75717/how-can-i-mix-itemize-and-tabular-environments#comment161963_75717
\documentclass[a5paper]{article}
\usepackage[T1]{fontenc}
\usepackage{amsmath}  % add packages you need here ...

\begin{document}
\begin{tabular}{lll}
    \labelitemi & És associativa:&$\quad(x+y)+z=x+(y+z)$\\
    \labelitemi & És commutativa:&$\quad x+y=y+x$\\
    \labelitemi & Té element neutre:&$\quad\exists 0\in\K : 0+x=x+0=x$\\
    \labelitemi & Té element invers:&$\quad\forall x\in\K\;\exists(-x)\in\K : x+(-x)=(-x)+x=0$\\
\end{tabular}
\end{document}

Here is a minor enhancement to this solution.

Using the array package you can define a new column type L which automatically inserts the bullet at the start of the column in which it is used:

Notes:

• This could also be done with the collcell package. This commented out in the code below, but that would only make sense if there was further processing you wanted to do with the entries in the first column.

Code:

\documentclass[a5paper]{article}
\usepackage[T1]{fontenc}
\usepackage{amsmath}  % add packages you need here ...

\usepackage{array}
\newcolumntype{L}{>{\labelitemi~~}l<{}}

%% Don't need above two lines if want to use `collcell` pacakage
%\usepackage{collcell} 
%\newcommand*{\AddBullet}[1]{\labelitemi~#1}%
%\newcolumntype{L}{>{\collectcell\AddBullet}{l}<{\endcollectcell}}

\newcommand*{\K}{\mathbf{K}}%  Not sure what \K is supposed to be

\begin{document}
\begin{tabular}{L l}
    És associativa:    & $\quad(x+y)+z=x+(y+z)$\\
    És commutativa:    & $\quad x+y=y+x$\\
    Té element neutre: & $\quad\exists 0\in\K : 0+x=x+0=x$\\
    Té element invers: & $\quad\forall x\in\K\;\exists(-x)\in\K : x+(-x)=(-x)+x=0$\\
\end{tabular}
\end{document}

There is no requirement for intermixing the two environments if you manage some of the box widths yourself. The following MWE shows an environment myitemize - based on itemize, that breaks across the page boundary and sets the width of an item (\myitemlen) if it's set at the beginning of the environment. Otherwise, it takes the natural width of each specified item.

\documentclass{article}
\usepackage[utf8]{inputenc}% http://ctan.org/pkg/inputenc
\newlength{\myitemlen}
\newcommand{\myitem}[1]{%
  \olditem\ifdim\myitemlen>0pt
      \makebox[\myitemlen][l]{#1}%
    \else
      #1%
    \fi}
\newenvironment{myitemize}
  {% \begin{myitemize}
   \begin{itemize}\let\olditem\item\let\item\myitem}
  {\end{itemize}}% \end{myitemize}
\newcommand{\K}{\mathbf{K}}
\begin{document}
\begin{myitemize}
  \item{És associativa:} $(x+y)+z=x+(y+z)$
  \item{És commutativa:} $x+y=y+x$
  \item{Té element neutre:} $\exists 0\in\K : 0+x=x+0=x$
  \item{Té element invers:} $\forall x\in\K\;\exists(-x)\in\K : x+(-x)=(-x)+x=0$
\end{myitemize}

\begin{myitemize}
  \settowidth{\myitemlen}{Té element neutre:\qquad}
  \item{És associativa:} $(x+y)+z=x+(y+z)$
  \item{És commutativa:} $x+y=y+x$
  \item{Té element neutre:} $\exists 0\in\K : 0+x=x+0=x$
  \item{Té element invers:} $\forall x\in\K\;\exists(-x)\in\K : x+(-x)=(-x)+x=0$
\end{myitemize}

\end{document}