Apache JMeter Thread Group Samplers
These are used to provide delay between various requests within same thread group or within various thread groups. Timers can be applied at test level, thread level, controller level, and request level. Some of the commonly used Timers are: Constant Timer: These timers are used to provide a constant delay.

JMeter Tutorial: Thread Group Timers

Apache JMeter Test Plan
It is considered as container of Jmeter. It contains various thread groups, controllers, and listeners using which the performance testing script can be created. Every Test Plan should have at least one thread group. Without a thread group performance testing will not be possible

JMeter Tutorial: Test Plan