Question

What are $f$-block elements $?$ Distinguish between lanthanoid and actinoid?

Answer 1

First we know that the elements in the periodic table are arranged into four blocks associated with the four sublevels $s,p,d,$ and $f$. This periodic table consists of groups and periods. Lanthanoid and actinoid are $f$-block elements in the periodic table.

Complete answer:
$f$-block elements are placed separately at the bottom of the periodic table and they are a subset of ${6^{th}}$ and ${7^{th}}$ periods. $f$-block elements are the elements in the periodic table in which last electrons enters into $f$ orbitals or simply, the elements whose $f$ orbital getting filled up by electrons are called $f$-block elements. These block elements also called as inner transition elements because they provide a transition in the ${6^{th}}$ and ${7^{th}}$ row of the periodic table which separates the $s$ block and the $d$ block elements.
Difference between lanthanoid and actinoid

Lanthanoid	Actinoid
Lanthanoids are non-radioactive except promethium	Actinides are radioactive elements.
Lanthanoid include the elements with atomic numbers starting from $57$ and ending at $71$	Actinoids include elements with atomic numbers starting from $89$ and ending at $103$.
Lanthanoids have a low tendency to form complexes.	Actinoids have a greater tendency to form complexes.
In lanthanoid, the last electrons enter into $4f$ orbitals.	In actinoid, the last electrons enter into $5f$ orbitals.
Binding energy of $4f$ orbital is higher.	Binding energy of $5f$ orbital is lower.

Note:
$f$-block elements are divided into two series corresponding to the filling up of $4f$ and $5f$ orbitals. The elements from $Ce$ to $Lu$ are $4f$ orbital series and $Th$ to $Lr$ are $5f$ orbital series. In each series there are $14$ elements. The first series of elements are called lanthanides and the second series of elements are called actinides.