Rémoulade is very similar to tartar sauce in that it's mayonnaise-based with a sprinkling of this, that, and the other in it. The differences are very slight, in my opinion. (Rémoulade has curry powder, for example.) To be honest, I'd use them just the same–with seafood, french fries, roast chicken, hamburgers, and sandwiches! This is another one of those things that I eyeball as I make it. You kind of just have to keep checking the consistency and taste. It should be a bit thick and chunky. I find that it's nice to have a homemade condiment handy. This'll be like your own secret sauce.
1 cup of mayonnaise
1 tablespoon or so of Dijon mustard
1 small red onion, finely chopped
10-15 small cornichons, finely chopped (I like the zippiness of the ones by the French brand Maille rather than sweet pickle relish)
1 tablespoon of small capers
A handful of mixed chopped herbs (parsley, chervil, tarragon, chives–some or all of the above)
A squeeze of fresh lemon juice
Salt and pepper to taste
Blend all of the ingredients together in a small bowl with a fork. There should be a distinct note of curry powder in the final rémoulade and its color should be tinted yellow.