Is this flash compatible with my camera?

Targus TG-DL20C flash, my camera is a Nikon D3100. Will it work with my camera?

Targus should be able to answer that question because we can't. Check with them or with Nikon.

If it says for Nikon DSLR's, yes. Otherwise I wouldn't put it on your hotshoe. You run the risk of damaging your camera with a flash that may have a voltage feedback.
Why would you buy Targus anything? They make a bunch of junk.

No. The DL20C is for Canon. You need the DL20N. (Spot the C and N in the model name?)

I've never used a Targus flash and could not say how good or bad they are so do some research.

I would add though you'd be better with the DL80N. You can get it for $35 from B&H (pre-owned I think).

I also had a look at the DL38N and it dawned on me that the flash is a generic model that is badged differently. I have used the equivalent of the DL38N model before but it was in the UK badged as a "Jessops" product. If I'm honest it did a fine job and I had no complaints. For the money anyway.

If you want something from a better known manufacturer then B&H sell a Sunpak unit for under $40. I've put the link in the source box for you.