The United States Department of Agriculture (USDA) defines, for its own internal use, a poultry Tender as any strip of breast meat from the bird indicated (e.g. a Chicken Tender.) This makes it the same as a Chicken Breast Strip.
The USDA defines a poultry Tenderloin as "the inner pectoral muscle which lies alongside the sternum of the kind of poultry indicated." Which is to say, a strip of muscle that runs along the inside of the breast, the part of the breast closest to the bone. It separates easily from the rest of the breast. It has a tendon running through it. The tendon is basically gristle that needs to be removed and discarded.
Neither "Chicken Tender" nor "Chicken Tenderloin", however, are official, enforced terms so there is much confusion. The USDA says "The terms tender and tenderloin have been used for a number of years for muscles from the breast without a clear-cut definition to distinguish between the two. The policy stated above appears to be what is being done in general practice."
A Chicken Tender can even be ground chicken that is formed into a strip and breaded and cooked. Such "Chicken Tenders" are often sold at restaurants these days. This is very far from the USDA's attempted definition, but again, the USDA definition isn't enforced.
Sometimes you will see Chicken Tenderloins being sold off cheaply, just to get rid of them, as the butcher doesn't want the fiddly work of removing the tendon. In other places, you will see them quite expensive, as the marketing cachet attached to the wording "tenderloin" starts to catch on so that a higher price can be charged for them. That is intended to be the trend.
Remove and discard the tendon from Chicken Tenderloins before cooking.