Toss Up: CONCATENATE and & both provide a simple means to join strings.
Excel provides two mechanisms for joining items together as a string: CONCATENATE and &. THat is, as something which will return TRUE from ISTEXT. They work the same but one works as an operator whereas CONCATENATE is a function. I’ve never had a preference for one over the other. Sometimes I get tired of holding shift and prefer using commas. Other times, I prefer the shorter function and use &.
When working with strings, the biggest issue is usually formatting. This is one of the more tedious aspects of strings. You often have to format all of your results using TEXT to prevent showing 99 decimal places. This is also usually required for returning dates in a string.
=CONCATENATE(A1,B2,C2)
=A1 & B2 & C2
The other major issue with strings is wanting to join together a range of cells into a single string. Excel provides no simple way to do this that takes the range as an argument. To do this, you will need to use VBA and a UDF which can join together the items. My bUTL addin has a couple of functions to help with that.