What are alkanes, alkenes, and alkynes?

Hydrocarbons are organic compounds which only consist of carbon (#C#) and hydrogen (#H#) as their elements.

Alkanes are a group of acyclic, saturated hydrocarbons. These are contain carbon - carbon (#C-C#) single bonds. The single bond is made up of one #sigma# (sigma) bond.
Their general formula is #C_nH_(2n+2)# . [Here, #n# stands for the number of carbon atoms].
Each atom has an #sp^3# hybridization in an alkane. The hydrogen atoms are always linked to a carbon atom.

#-># Examples : Methane (#CH_4#, where #n=1#); Butane* (#C_4H_10#)

Alkenes and alkynes are unsaturated hydrocarbons.

Unsaturated hydrocarbons are those in which you have atleast one carbon - carbon double bond (#C = C#), or atleast one carbon - carbon triple bond.

In the case of alkenes , they have atleast one carbon- carbon double bond. Their general formula is #C_nH_(2n)#. From the formula, you can notice that alkenes only have 2 #H# atoms lesser than the corresponding alkane.
The carbon - carbon double (# C = C#) bond consists of one #sigma# and one #pi# (pi) bond.

# -># Examples : Ethylene (#C_2H_2#) or ethene; Pentene (#C_5H_10#).

Alkynes are unsaturated hydrocarbons which have atleast one carbon- carbon triple bond. Their general formula is #C_nH_(2n-2)#. You can see that these have 2 hydrogen atoms less than their corresponding alkane.

#-># Examples : Acetylene (called ethyne) i.e. #C_2H_2#;
Propyne (#C_3H_4#).

I hope this helps!