I read the wikipedia page about sequence alignment and sequence assembly but I have not been able to find any difference between the two. What is the difference between sequence alignment and sequence assembly? If there is no different why are the terminologies different?
It is done for checking sequence similarity between two or more different sequences. This will give information about how two sequences are different, what is their evolutionary relationship, which residues are conserved etc. Take a look at following sequence alignment between different sequences. (Image courtesy: Wikimedia Commons)
You can see conversation of different amino acid residues in variety of organisms like human, mouse, rat etc.
It is done for creating long consensus sequence from short fragments of same sequence. Generally you would do sequence assembly after sequencing some genome or large DNA fragment. Take a look at following representative image of sequence assembly (Image courtesy: Wikimedia Commons)
Here black sequence is your original sequence which you wanted to sequence. However because of technical difficulties you can't sequence all at once. Hence you fragment and sequence different fragments which are shown in red, blue and green). Looking at overlapping region between different sequenced fragments you calculate consensus. Potentially problematic repeats are shown above the sequence (in pink) Here you are ideally sequencing only one large sequence and assembling it.
The two tasks are quite different.
Aligning means comparing sequences to find differences between them.
Assembling means that you take a bunch of short sequences and try to stitch them back together to reconstitute the original DNA sequence.
Note about the input of the two processes:
The sequences that enter into an alignment have (supposedly) already been cleaned up. Every base is correct/accurate.
The sequences that enter into an assembly are 'dirty' and inaccurate, because it is highly likely that the sequencing machine will generate 'bad' bases at the ends of the sequence. The user will have to manually inspect and clean up the sequences (even though today there are smart assembly programs that can fully automate the process reducing the time from 10 minutes per contig to basically nothing (under 1 second)). It is preferably that multiple overlapping sequences are used for redundancy. This way 'bad' bases are discovered and eliminated.
Once the assembly job is done, the clean contig can be used for further operations (such as alignment).
Not necessarily. A good analogy for differentiating the two sequencing strategies (not to be confused with sequencing technologies i.e. illumina, smrt, oxford nanopore etc...) would be a puzzle. In the case of sequence alignment, you would have a bunch of puzzle pieces i.e. your reads and a picture of the finished/completed puzzle i.e. your reference genome to guide you in putting together the puzzle. In sequence assembly, you only have the puzzle pieces and no picture, no reference genome. This makes assembly a much harder task.
In the first case, you would compare the N reads against only 1 reference genome. In the second case you would compare N reads against N "quasi" reference genomes.