Exactly how you deal with this depends on exactly what you are doing: if you are entering this directly via SQL Sever Management Studio then just double up the quotes:
INSERT INTO demo VALUES('This is Description. Assessment''s. ')
(You should list the field names you are trying to insert into though - it can help prevent errors later)
However, if you are doing this via code from VB or C# for example, then you really should use a parameterized query to do the insert, and it will sort it out for you. (Concatenating strings to form an SQL query is very dangerous - it leaves you wide open to an SQL Injection Attack, which could damage or destroy your database).