Are you asking about the difference between First, Second, and Third Degree Felonies?
A First Degree Felony is punishable by 5 years to 99 years or Life and includes such crimes as murder, aggravated robbery, aggravated sexual assault, aggravated kidnapping, high amount of drug cases, and many others.
A Second Degree Felony is punishable by 2 years to 20 years in TDCJ and includes offenses like Robbery, Aggravated Assault, Sexual Assault, and others.
A Third Degree Felony is punishable by 2 years to 20 years in TDCJ and includes offenses like DWI-3rd Offender, Assault-Family Violence (2nd), lower level drug cases, and others.
There is also another felony degree called the State Jail Felony, which is punishable by 6 months to 2 years in a State Jail Facility. This includes most less than a gram drug charges, and Theft-3rd Offenders.
All of the different degrees can be "enhanced" with previous convictions with pen time. For instance, a second degree charge where the Accused has a prior trip to TDCJ for a 1st, 2nd, or 3rd degree felony can be filed as a 1st Degree felony.
There are many different scenarios that depend on how much criminal liability a person is looking at, and you should consult an attorney to see what facts apply to the case you are interested in.