A 'T' intersection is a three way intersection. You can either go left or right. The street is shaped like the letter 'T' so people call it a 'T'. Sometimes they don't call it a 'T'. Other names for a 'T' intersection you might hear are more descriptive. Here is a list.
'When the road ends...' 'When you can't go anymore...' 'Three way intersection...'
This isn't a direction terminology, but dead end is a road that has only one entrance. That means the only way out is the way you came in. All dead end streets will have a sign at the entrance with the word 'Dead end'.
If you are going to a location that is in a dead end, someone might use this word.