I am using Greek, English babel. For some reason when I generate the pdf there are different fonts for greek and different for english. How can I use the same fonts and are the fonts machine dependent? See image:

This answer demonstrates some of the possibilities described in the LaTeX Font Catalogue. The sample text is from Lou's answer. The sample maths is from kerkis's documentation.
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[greek, english]{babel}
\usepackage[T1]{fontenc}
% uncomment as desired
% \usepackage{kmath,kerkis}
% \usepackage{gfsartemisia}
% \usepackage{gfsartemisia-euler}
\begin{document}
\selectlanguage{greek}
Φιμ αδ σαπιενθεμ εξπετενδα περσιπιθυρ. Αδ συμμο περσιπιθ ελεστραμ φις. Ευ σαυσαε περσιπιθυρ σιθ, ηις αυγυε ασομμοδαρε ευ. Ιν ορναθυς ινσωλενς δεσερυντ περ, εσθ ει αδμοδυμ ινσωλενς αντιωπαμ.
\selectlanguage{english}
This text will be in English. The elements within this block of text will also be set in the right language.
\[
\sum_{i=1}^{+\infty} \frac{1}{n^2} = \frac{\pi^2}{6}
\]
\end{document}
This is the solution mentioned by the OP in comments on the original question:

Pulls in some elements from Nimbus Roman No9:


\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[greek, english]{babel}
\usepackage[T1]{fontenc}
% uncomment as required
% \usepackage{dejavu}
% \usepackage[default]{gfsbodoni}
% \usepackage{gfsdidot}
\begin{document}
\selectlanguage{greek}
Φιμ αδ σαπιενθεμ εξπετενδα περσιπιθυρ. Αδ συμμο περσιπιθ ελεστραμ φις. Ευ σαυσαε περσιπιθυρ σιθ, ηις αυγυε ασομμοδαρε ευ. Ιν ορναθυς ινσωλενς δεσερυντ περ, εσθ ει αδμοδυμ ινσωλενς αντιωπαμ.
\selectlanguage{english}
This text will be in English. The elements within this block of text will also be set in the right language.
\end{document}



To combine fonts it is useful to define commands to combine font and language switches:
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[greek, english]{babel}
\usepackage[T1]{fontenc}
\usepackage{baskervald}% Latin Baskerville substitute; Libre Baskerville would be an alternative
\usepackage{gfsbaskerville}% Greek Baskerville
\newcommand{\switchtogreek}{%
\fontencoding{LGR}\fontfamily{gfsbaskerville}\selectfont\selectlanguage{greek}}
\newcommand{\switchtoenglish}{%
\fontencoding{T1}\fontfamily{ybv}\selectfont\selectlanguage{english}}
\begin{document}
\switchtogreek
Φιμ αδ σαπιενθεμ εξπετενδα περσιπιθυρ. Αδ συμμο περσιπιθ ελεστραμ φις. Ευ σαυσαε περσιπιθυρ σιθ, ηις αυγυε ασομμοδαρε ευ. Ιν ορναθυς ινσωλενς δεσερυντ περ, εσθ ει αδμοδυμ ινσωλενς αντιωπαμ.
\switchtoenglish
This text will be in English. The elements within this block of text will also be set in the right language.
\end{document}

International language support may be helpful: see this page for Examples.
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[english, greek]{babel}
\usepackage[T1, T2A]{fontenc}
\begin{document}
\begin{abstract}
Φιμ αδ σαπιενθεμ εξπετενδα περσιπιθυρ. Αδ συμμο περσιπιθ ελεστραμ φις. Ευ σαυσαε περσιπιθυρ σιθ, ηις αυγυε ασομμοδαρε ευ. Ιν ορναθυς ινσωλενς δεσερυντ περ, εσθ ει αδμοδυμ ινσωλενς αντιωπαμ.
\end{abstract}
\selectlanguage{english}
This text will be in English. The elements within this
block of text will also be set in the right language.
\begin{abstract}
A brief description of the main subject to be
explained in the entire document.
\end{abstract}
\selectlanguage{greek}
Θε ασυμ μελιυς νονυμες φιμ, κυεμ περσεσυτι ρεπυδιανδαε φιξ εα. Δυο εα ιυσθο ρεπριμικυε, αλιι αεθερνο σριβενθυρ ατ πρω. Σιθ σονεθ νιηιλ ανιμαλ νε. Μει αδ βρυτε πωσε δισθας. Ιδ μεα χινς πριμα, δυο εα φερρι φασιλις ιμπερδιετ, ατ σασε φοσεντ λαβωραμυς υσυ. Εα φις κυεμ απειριαν.
\end{document}

xelatexorlualatexand use any Opentype font on your system that has greek and latin letters. For instance,Minion ProandMyriad Pro, that come with Adobe Reader, have monotonic greek, cyrillic and latin alphabet. The maths fonts are another problem. – Bernard Apr 06 '14 at 14:07newtxoffers the possibility to use Linux Libertine as a Times-like font. Some fonts from thegfsbundle also have both greek ans latin fonts, but it doesn't offer math support. – Bernard Apr 06 '14 at 15:04ls <path to texmf tree>/fonts/type1/public/tex-gyre/confirms what ctan tells me ;). – cfr Apr 06 '14 at 15:09