difference between thermocouple types

The difference between the types is type of metal used.

B is Platinum-30%Rhodium and Platinum-6%Rhodium
J is iron and constantan
K is Chromel Nickel-Chromium and Alumel Nickel_Alumel
N is Nicrosil Ni-Cr-Si and Nisil Ni-Si-Mg
R is Platinum-13%Rhodium and Platinum
S is Platinum-10%Rhodium and Platinum
T is Copper and Constantan
There are many good internet sites to get the information. The two I mostly use are watlow.com and omega.com.
If you have to ask that question you should not be playing with them.
Material composition, measurement range.
For eXample, j is iron vs constatntan, k is chromal vs alumal etc etc. These are probably the 2 most used types but are not suitable for very high or low temperatures. I do not use j as iron rusts.
How about n? Nicrosil vs nisil. Used in nuclear power stations.
Difference, is solely due to metals and metal alloys used. For example thermocouple S and R use nobel metals like platinum and rhodium. They are very accurate and are relatively expensive. The indicators they use are expensive.

K type is one of the cheapest and reliable use chromel aluminum.

Refer to Omega.com for more details on t/c.