The front nose is different.
The biggest difference is the engine's. Each manufacture, Ford, Chevy, Dodge and Toyota are required to design there own engine blocks etc. within the NASCAR rule book.
Toyota builds most of the engines for the Toyota teams through there TRD department. TRD stands for Toyota Racing Development.
Penske Racing receives there Dodge engines in pieces and they build them themselves in there engine shop.
A lot of Ford engines are built by Roush/Yates for themselves and other Ford teams.
A lot of Chevrolet engines are built by Hendrick and Earnhardt/Childress, otherwise known as ECR, for themselves and other Chevy teams.
All 4 car makers give ongoing research and development support.
The rest of the car is pretty much the same. However, in 2013, there letting the car makers shape the body's to more resemble the real thing again like the old days which is awesome! No more flat boring sides. All the body lines are contoured.
Here's the 2013 NASCAR Dodge Charger: http://www.google.com/imgres?um=1&hl=en&biw=1280&bih=653&tbm=isch&tbnid=Oi9_HVn2cOFtJM:&imgrefurl=http://www.inflexwetrust.com/2012/03/12/auto-talk-2013-nascar-dodge-charger-sprint-cup-car-unveiled-in-las-vegas/&docid=xd_5ott0PTbbPM&imgurl=http://www.inflexwetrust.com/wp-content/uploads/2012/03/IFWT_2013NASCARDodgeCharger.png&w=628&h=431&ei=qwDUT5SAHYuk8gS9l-nBAw&zoom=1&iact=hc&vpx=189&vpy=129&dur=408&hovh=169&hovw=239&tx=116&ty=91&sig=114057761190957881805&page=4&tbnh=144&tbnw=192&start=55&ndsp=20&ved=1t:429,r:10,s:55,i:288
Here's the 2013 NASCAR Ford Fusion: http://www.google.com/imgres?um=1&hl=en&sa=N&biw=1280&bih=653&tbm=isch&tbnid=QRdI8JrvqyI5KM:&imgrefurl=http://www.autobytel.com/ford/fusion/news/nascar-s-2013-ford-fusion-helps-put-the-stock-back-in-stock-car-109943/&docid=UvGnZMRouJmIZM&imgurl=http://img.autobytel.com/car-reviews/autobytel/2013_ford_fusion_nascar2.jpg&w=376&h=282&ei=fwHUT7uYJYyQ8wS4uaXjAw&zoom=1&iact=hc&vpx=401&vpy=175&dur=581&hovh=194&hovw=259&tx=124&ty=101&sig=114057761190957881805&page=3&tbnh=143&tbnw=191&start=35&ndsp=20&ved=1t:429,r:6,s:35,i:209
Here's the 2013 NASCAR Toyota Camry: http://www.google.com/imgres?um=1&hl=en&biw=1280&bih=653&tbm=isch&tbnid=s4AvwYyOxDvV9M:&imgrefurl=http://www.jayski.com/teams/toyota.htm&docid=Zj8kgB5TQsYfKM&imgurl=http://www.jayski.com/schemes/2013/2013toyota-camry-jgr-nose.jpg&w=700&h=639&ei=3QHUT_J0kajyBLm2xd8D&zoom=1&iact=rc&dur=568&sig=114057761190957881805&page=1&tbnh=145&tbnw=172&start=0&ndsp=15&ved=1t:429,r:12,s:0,i:118&tx=89&ty=74
Chevrolet has yet to introduce there new 2013 model yet but it is confirmed it won't be the Impala. There bringing out a new car called the 'SS Sedan' that's somewhat of a continuation of the popular V8 rear wheel drive Pontiac G8 but in Chevy form!