Greek in tex.

latex.tex

%encoding utf-8
\documentclass[12pt]{article}
\usepackage[greek,english]{babel}
\usepackage[utf8x]{inputenc}

\begin{document}
{\selectlanguage{greek}Ελληνικά και} english
\end{document}

pdflatex.tex

%encoding utf-8
\documentclass{article}

\usepackage[greek,english]{babel}
\usepackage[utf8x]{inputenx}
\usepackage[LGR,T1]{fontenc}
\usepackage{textcomp}

\newcommand{\greek}[1]{{\selectlanguage{greek}#1}}

\begin{document}
Engilsh and \greek{ελληνικά}
\end{document}

xelatex.tex

% encoding utf-8
\documentclass[12pt,a4paper]{article}

%Greek
\usepackage{xgreek}
\usepackage{fontspec}
\usepackage{xunicode}

\setmainfont{DejaVuSerif}


\begin{document}
Ελληινκά και English.
\end{document}