What Delphision said.

That said, for those looking at current or last year's software, if you have only one or two return to prepare within your immediate family, your taxes are relatively simple, and you expect to be able to go through interview from start to finish without having to move around much or see the actual forms, the TT Online Free (if you qualify), Student, or Standard edition may be suitable for you.

Otherwise, I would strongly recommend that you consider the limitations of the Online versions before choosing one (applies to all versions):

• No access to forms for either entry or viewing during preparation (pdf copy available upon completion), interview process only.
  
• Tax file cannot be downloaded or imported into desktop versions or other tax software, for either current-year returns or transfer of prior-year data.
  
• Tax files are accessible only for the current and previous year's return.
  
• Many situations will require you to upgrade, without which you cannot continue.
• The choice to upgrade is irreversible - downgrading can only be done by creating a new return.
• TT Online accounts cannot be deleted (although you can replace your personal information with gibberish).
• Charged on a per return basis.
  

For all desktop versions Standard and above:

• One-click switching between Forms Method (direct access to all forms, schedules, and worksheets) EasyStep interview.
  
• Tax files are saved on your computer.
• Prior-year tax data can be transferred to TT Online and other tax software. (Current year files can only be opened by TT desktop versions.)
• Upgrading to Premium or Home & Business may be recommended, but is not required to continue - Standard can be used for all tax situations. The difference is that the higher end versions will offer additional features and guidance that may be helpful to many people. (I would always urge those whose situation prompts an upgrade recommendation to take it for at least the first year to become familiar with the added benefits.) Upgrades are irreversible.
  
• No personal information or tax data is stored in the cloud.
• Up to 20 returns (lower limits depending on income) per installation.
  

The above appears obviously biased in favour of desktop versions, so to be fair I will repeat that for relatively simple tax situations, Online versions may be preferable. They are also available to all operating systems, while desktop versions are only available for Windows.

Most importantly, do you own research and ask the questions that are relevant to you before making a choice. Caveat emptor.