Noob here, learning lots everyday. I googled my question, but didn't come up with anything. Probably due to my lack of correct terminology.

Why are some resistor values expressed as a decimal i.e., .1, .33 and others are expressed using an R 0R3, 0R5, 2R7?

 

It is very easy to miss a decimal point on components and printed documents.
The convention adopted is to place R, K, or M where the decimal place belongs.

0R3 = 0.3Ω
2R4 = 2.4Ω
4K7 = 4.7KΩ
2M2 = 2.2MΩ

Similarly, for capacitance
5p6 = 5.6pF
0n1 = 100pF = 0.1nF
4n7 = 4.7nF

 

Perfect, Thank you. If I understand correctly, 1MR25 would be 1.25MΩ?

 

Additionally, when SMD parts are marked it indicates both value and that it is a resistor.

 

Perfect, Thank you. If I understand correctly, 1MR25 would be 1.25MΩ?

Or 1M25 is 1.25MΩ. The convention is to use one of the letters (depending on the magnitude of the value) in place of the decimal point. Hence, 1MR25 is redundant. Do you mean 1.25Ω or 1.25MΩ?

 

You may also find 2K47 or 300R for 2470Ω and 300Ω
Max.

 

Or 1M25 is 1.25MΩ.

To the TS:
Be sure you are aware of the letter case for M, since 1M25 is 1.25 megohm and 1m25 is 1.25 milliohm.
(Numerous newbies have had their Spice simulations puzzlingly fail because Spice doesn't pay attention to letter case, interpreting all letters as lower case, so M becomes milli. You must use meg for mega.)

The case of letters k, n, and p however, makes no difference, although by convention they are lower-case.