.sty

renewcommand{sfdefault}{phv}

renewcommand{sfdefault}{PTSansCaption-TLF}

renewcommand{rmdefault}{...}
renewcommand{ttdefault}{...}

fontfamily{<familyname>}selectfont

newcommand*{myfont}{fontfamily{<familyname>}selectfont}

{fontfamily{<familyname>}selectfont ...}

{myfont ...}

begingroup
  myfont ...
endgroup

newenvironment{myfont}{fontfamily{<familyname>}selectfont}{par}

begin{myfont}
  Some text in the new font.
end{myfont}

DeclareTextFontCommand{textmyfont}{myfont}

Text in the default font. textmyfont{Text in the new font.} Again text in the default font.

documentclass{article}
usepackage{fontspec}
setmainfont{<any font on your system>}
begin{document}
...
end{document}

setmainfont[<font features>]{<name of font>} % sets the roman font
setsansfont[<font features>]{<name of font>} % sets the sans font
setmonofont[<font features>]{<name of font>} % sets the monospace font

newfontfamilymyfont[<font features>]{<name of font>}

defaultfontfeatures{Ligatures=TeX} % makes this a feature for all selected fonts

setmainfont{Linux Libertine O}
newfontfamilymyfont{Linux Libertine O}

{myfont ...}

begingroup
  myfont ...
endgroup

newenvironment{myfont}{myfont}{par}

begin{myfont}
  Some text in the new font.
end{myfont}

DeclareTextFontCommand{textmyfont}{myfont}

Text in the default font. textmyfont{Text in the new font.} Again text in the default font.

export OSFONTDIR="/usr/local/share/fonts;$HOME/.fonts"

mtxrun --script fonts --reload

mtxrun --script fonts --list --all

ams                   wncysc10              wncysc10.afm
amsbold               wncyb10               wncyb10.afm
amsmedium             wncysc10              wncysc10.afm
antpoltbold           antpoltbold           antpolt-bold.otf
antpoltbolditalic     antpoltbolditalic     antpolt-bolditalic.otf
antpoltcondbold       antpoltcondbold       antpoltcond-bold.otf
antpoltcondbolditalic antpoltcondbolditalic antpoltcond-bolditalic.otf
antpoltconditalic     antpoltconditalic     antpoltcond-italic.otf
antpoltcondregular    antpoltcondregular    antpoltcond-regular.otf
antpoltexpdbold       antpoltexpdbold       antpoltexpd-bold.otf

mtxrun --script fonts --list --pattern=deli* --all

delicious             deliciousbold         Delicious-Bold.otf
deliciousbold         deliciousbold         Delicious-Bold.otf
deliciousbolditalic   deliciousbolditalic   Delicious-BoldItalic.otf
deliciousheavy        deliciousheavy        Delicious-Heavy.otf
deliciousitalic       deliciousitalic       Delicious-Italic.otf
deliciousmedium       deliciousitalic       Delicious-Italic.otf
deliciousnormal       deliciousroman        Delicious-Roman.otf
deliciousregular      deliciousheavy        Delicious-Heavy.otf
deliciousroman        deliciousroman        Delicious-Roman.otf
delicioussmallcaps    delicioussmallcaps    Delicious-SmallCaps.otf

definefont [delicious] [name:deliciousroman]

definefont [delicious] [name:deliciousroman]

starttext

start
    delicious
    input knuth
stop

input knuth

stoptext

ttfamily http://www.ctan.orgnormalfont

{ttfamily http://www.ctan.org}

begin{ttfamily}
statement 1;
statement 2;
...
statement n;
end{ttfamily}