Gene is present on chromosomes so the alleles should be same as gene but what is difference between them and what is traits?
An allele is a variant of a gene. To understand what that means, the easiest is to consider a simplified example...
There is a gene for eye color. Eye color is a trait (aka. phenotypic trait). In the human population, there are different variants at the gene for eye color. These variants are called alleles. There is the allele 'blue eyes', the allele 'green eyes', the allele 'brown eyes', etc...
Above I wrote
An allele is a variant of a gene. It would actually be more correct to say
An allele is a variant at a locus but I did not want to introduce one extra word too early. A locus is a region of arbitrary size on a chromosome.
Genetic variation and phenotypic variation
If there are more than one allele at a given locus, then there is genetic variation in the population at this locus. This genetic variation can cause variation in various phenotypic traits.
Considering again the example of the gene for eye color, the fact that there are different alleles at the gene for eye color causes that there is variation in eye color in the population.
Although it may be too advanced for you, you can learn more about these concepts and how it related to heritability in this post.
Source of information
You can also have a look at the wikipedia articles
Or have a look at an intro course such as these ones at Khan Academy.