I did few recordings on karaoke tracks by plug in a condenser mic with Audacity directly. I would like to know what will differ if I use an audio interface in between audacity and condenser mic.
Audacity is software. A microphone is hardware. Your statement does not make sense.
Which version of Windows?
Which version of Audacity? (look in “Help > About Audacity”)
What make/model of mic?
How is the mic connected to your computer?
What sort (make and model) of audio interface are you considering?
Computer mics are high impedance unbalanced (2-wire) and computer electret condenser mics run from 5V supplied by the soundcard. A computer mic won’t work with an interface.
An audio interface ([u]example[/u]) works with stage/studio mics which are low impedance balanced (3-wire) with an XLR connector. A stage/studio mic won’t work with a soundcard. Studio condensers run from 48V phantom power, supplied by the interface (or preamp or mixer). Some stage condensers run from a built-in battery. Dynamic mics don’t require power.
You’ll generally get much better quality with a stage or studio mic and an interface. The most common type of studio mic (used for vocals and almost everything else) is a “large diaphragm directional condenser”.
Quality also depends on having a good quiet “studio” with good acoustics (generally with “dead” acoustics), good mic position, and of course a good performance.
An alternative is a “studio style” USB mic (AKA “podcast mic”).
If you get a podcast mic, look for one with a built-in headphone jack and zero-latency monitoring. A gain control is also a good feature.
If you get an interface, I also recommend one with zero-latency monitoring. Otherwise, the monitoring path goes through the computer and you can get latency (a delay) in your headphones making it difficult to perform.
Your choice of recording software does NOT affect sound quality… The software just “captures” the digital audio stream and sends it to your hard drive. (If you start applying effects, software can make a difference, or if you are doing multitrack recording & mixing you may need higher-end software.)