Both of them seem to mean a second, according to both wiktionary and linedict.
That is a question common to 134% of Chinese students (yeah, I just made up a nonsense number), and I personally think it's one of the most difficult issues in the Chinese language.
About 50% of the words are elastic in Chinese (check the concept here How to deal with elasticity/flexibility in HSK vocabulary?)
To put it simple (very very simple), Chinese have words coming in one and two syllables that mean exactly the same. There are many theories trying to explain why that happens, and one of them (which I think is the easiest to understand) is about homophony causing ambiguity.
I'll give an example with the word 分 [fēn]. If someone says:
One wouldn't be able to tell if he/she is saying "I still have one minute" or "I still have one cent", since 分 can mean both. To solve the ambiguity the words 钟 and 钱 should be added:
我还有一分钟。 I still have one minute. ("one 分 of hour")
我还有一分钱。 I still have one cent. ("one 分 of money")
So, the difference between 秒 and 秒钟 it's just that 秒钟 is the complete word for "second". But if you have context, you don't need the whole word. For example:
A: 多少钱？ How much is it?
B: 一分。 One fen. (the context clears up that it's a "fen of money")
If we're only talking about the English word second, then 秒钟 and 秒 don't really have any difference.
Perhaps you could think of 秒 as
sec, as in
give me sec or
I'll be there in a sec & 秒钟 as
If we check out MDBG's definition of 秒 we can see that it also includes other things beside a measurement for time, like:
arc second (angular measurement unit)
Otherwise for general purposes: 秒钟 is just the lengthened form of 秒 & 秒 is just the shortened form of 秒钟.